Among the papers found in Portuguese writer Fernando Pessoa’s possession after his death in 1935 was an English-language book he finished but never bothered to publish. Lisbon: What the Tourist Should See was written as part of an imagined public-relations campaign for his beloved city, which Pessoa felt was neglected in favor of places like London and Paris. The book, finally released in 1992, highlights the greatest hits of the Portuguese capital: the 16th-century monastery that is now a UNESCO World Heritage Site, the nearby town famous for its eclectic castles and mansions, and the winding fisherman’s quarter where locals go to hear fado singers perform.

These sites still exist, though in a universe distinct from Pessoa’s. In a sense, he got his wish: Lisbon has become one of the most popular tourist destinations in Europe, and is, as a result, in on the cusp of a massive physical transformation. Restaurants, luxury hotels, and “concept” boutiques are springing up all over the city. Cranes mar the views from hillside miradouros and rooftop bars. During peak season, tuk-tuks and double-decker sightseeing buses prowl the city for customers. Cruise ships loom in the harbor. English is the lingua franca.

This is in large part to accommodate the record 12.7 million people who visited the city last year, and the even larger numbers of those projected to come. If trends persist, locals worry that the city will turn into Paris—only without Eurodisney in the suburbs.

In recent years, “overtourism” has become a travel-industry buzzword, a phenomenon created by the perfect storm of budget airlines, short-term rental websites, and social media. (“You know, the Instagram island,” an Australian backpacker said to me recently when describing the Greek island of Santorini.) The problem is especially acute in Europe, which opened the floodgates to short-term travel in 1997 by liberalizing its transportation market. Suddenly, dozens of new, low-cost regional airlines began advertising pocket change prices, and average trip length plummeted. Formerly sleepy cities became crash pads for weekenders, cobblestone streets echoed with the sounds of clicking heels and rolling suitcases, and urban infrastructures strained under the pressure.

While Lisbon is not as fully saturated as, say, Barcelona, the threat is on the horizon. It’s no longer a novelty to see bachelorette parties in identical raunchy T-shirts careening down the avenues, or four-euro mojitos hawked on sidewalks. The foreigners are coming; the question is whether the Portuguese capital can effectively manage them.

There are no precise standards for gauging overtourism. One thinks of Justice Potter Stewart’s famous definition of pornography—you know it when you see it. And you see it when Europeans and Americans flood downtown areas, and intoxicated visitors relieve themselves in public. Tourists in Europe are hard to avoid. Their presence wouldn’t be so vexing if they weren’t also good for local economies: Nearly 17 percent of Portugal’s GDP came from tourism in 2017, and that number is expected to rise by more than 5 percent this year. In Spain, tourism made up 15 percent of the GDP, expected to increase 3 percent; and in Greece, these figures are 20 and 5 percent, respectively. Still, local government regulation has not caught up to the market, and in hot spots like Barcelona, some officials claim that it costs the city more to manage visitors than it earns from their trips.

Barcelona has emerged as a case study in handling overtourism, which it has dealt with since hosting the Olympics in 1992. The city gets 32 million visitors every year to its 1.6 million residents. The number of beds available to book has quadrupled since 1990, and rents have followed suit, with real-estate prices in Barcelona now 50 percent higher than in the rest of the country. Lucila Mallart, a historian who lived in the middle-class Barcelona neighborhood of Eixample before moving to Girona, a smaller Catalan city about an hour away, says, “There is a general feeling of ‘normal’ people fleeing the city. Most of our friends who still live there are the ones who bought a flat 10 years ago, the rest have left because now they cannot afford to rent or buy. Many have moved to neighboring towns, which were traditionally working class.”

Eixample is far from gentrified in comparison to areas like El Born or the Barri Gotic, which long ago replaced corner stores and workers’ bars with champagne caves and luxury outlets, but nevertheless, residents feel the strain. Back in 2006, locals would know when it was time for dinner when drunken British and Northern European tourists began stumbling from bars. Since then, the problem has intensified. Las Ramblas, the central thoroughfare that was once the pride of the city, is so clogged with street performers, pickpockets, and tourists that Catalans go out of their way to avoid it.

Other locals have taken a more confrontational approach: Anti-tourist graffiti punctuates popular neighborhoods, and earlier this summer, members of a youth group associated with the Catalan independence party attacked a tourist bus with smoke bombs and banners. The incident was picked up by the Spanish-language press as an instance of turismofobia, yet it illustrates the sensitive dynamics that underlie the issue. What looks like anti-tourist agitation to one person may come across as classism, racism, or xenophobia to another—particularly if that person doesn’t fit the stereotype of a “guiri”: a pale, Anglo tourist with an affinity for booze and money to burn.

Public feelings about tourism have long been mixed. Back in the 1930s, says Harold Goodwin, director of the Institute of Place Management at Manchester Metropolitan University, there was documentation of hard-partying tourists destroying Greek fishing villages. “It’s always been true that people go abroad to behave badly,” he observed. In the 1970s, tourism sociologists developed the “Irritation Index” to gauge residents’ feelings about unwanted visitors, and in 1975 Swiss scholar Jost Krippendorf published The Landscape Eaters, a now-canonical text about the ecological consequences of excessive tourism in the Alps. At the same time, people recognize that wanting to manage the problem isn’t the same as wanting to drive away visitors. A 2017 poll conducted by the local government found that although 60 percent of people believed that Barcelona had exceeded its ability to host tourists, more than 83 percent thought that tourism was, on balance, a good thing for the city.

In response to public grumbling, now amplified on social media, policy-makers across the continent have adopted a range of measures to preserve quality of life: banning cruise ships (Venice), coating walls in pee-resistant paint (Hamburg), cracking down on Airbnb (Berlin), and outlawing selfie sticks (Milan). In Andalucía, where local governments are weighing a ban on bachelor and bachelorette parties, some say the roving gangs of screaming foreigners are no worse than street festivals or soccer celebrations, while others recall more traumatic encounters.

“After seeing a husband-to-be in Granada riding a donkey while handcuffed to a dwarf dressed as a gypsy…I thought I had seen everything,” wrote a tour guide from the northeastern Spanish city of Zaragoza on Facebook. That was, “until the moment when people on both sides of the street began to sing.”

Efforts to minimize mass tourism often focus on carrying capacity, or the notion that having more than a certain number of people in a confined space will be culturally and environmentally disruptive. Andreas Papatheodorou, a sociologist at the University of the Aegean who studies the implications of transportation on tourism, notes that some policy-makers are pushing a “quality over quantity” approach—targeting visitors who can afford to stay longer and spend more.

One way of doing this is by levying a general tourism tax on individuals; another is through accommodation taxes on lower-quality hotels and hostels. Visitors in Rome pay between three and seven euros a night simply to stay in the city, while Croatia’s “sojourn tax” imposes higher fees on tourists who visit during peak season, similar to congestion pricing on cars, in order to spread out tourism over the year.

The problem, of course, is that such measures have a disproportionate effect on lower-income travelers, which illustrates another uncomfortable reality: People rarely get angry about extremely wealthy travelers, who are generally insulated from the day-to-day life of a city. Moreover, this plays out on a geopolitical scale, as less-affluent countries are more affected by travel taxes than wealthier ones. According to a 2017 report commissioned by the EU, the measures have the sharpest impact on coastal destinations in Southern Europe, where steep competition means that even an incremental price increase of three or four euros can deter potential visitors.

Furthermore, not everybody agrees that the problem is space. “Many academics are going on and on about the carrying capacity of a place,” says Goodwin, who criticizes this emphasis as misplaced. “You can accommodate very large numbers of tourists with no impact at all if you manage them carefully,” he says.

Disneyland “does that very well,” Goodwin says, explaining that the theme park exemplifies sustainable tourism—any system in which a visitor’s admission covers the full cost of taking care of them in that destination, from dealing with litter to sewage to food. Still, the fact remains that it’s much easier to build a tourist trap like Disneyland from scratch than it is to transform a living, breathing city into a place that tourists can sustainably visit.

Barcelona’s strategy has been several years in the making. In 2015, after getting elected on a platform of reducing mass tourism, socialist Mayor Ada Colau declared a yearlong moratorium on new hotel construction and began fining and shutting down illegal Airbnb rentals. Since then, her administration has focused on better integrating tourism into city infrastructures. Under the Special Tourist Accommodation Plan, Barcelona grants new licenses to tourist accommodations—from hotels to Airbnb listings—based on neighborhood congestion, with an eye towards more evenly distributing visitors throughout the city. The plan, which has been heavily criticized by the hotel industry, is set to go into effect in 2019.

Harold Goodwin, who has worked on sustainable-tourism initiatives in Barcelona, believes that the city has done more than anywhere else in the world to test out different methods of responding to mass tourism, and has adopted what he calls “a remarkably progressive” approach to the issue. “They’ve refused to scapegoat the tourist,” he says. “They take the view that the problem is managing large numbers of people in an urban space whether they’re business tourists, leisure tourists, or students.”

Part of the insight in Colau’s strategy is in differentiating between problems of housing and tourism. Since 2015, Barcelona has responded to real-estate pressures from what Goodwin calls a “whole government perspective.” After separating tourism promotion from tourism management, the municipal authority created a cross-departmental working group to deal with tourism as a whole, meaning that the inspectors responsible for taxing short-term rentals are not the same people who respond to late-night calls from neighbors about Airbnb orgies. In making these distinctions, the aim is to adjust regulation accordingly, so (remaining) locals won’t be priced out of their homes by visitors able to pay significantly more.

Additionally, the city is dedicating more resources to the study and management of tourism. None of these methods are new, but the lesson of Barcelona, Goodwin says, is that there’s no one single solution to overtourism. In other parts of the world, “de-marketing” and even removing alluring place names from visitor maps have proven effective in changing behavior. (Goodwin confessed that he once did this while working on a park project in the Philippines. It worked splendidly.)

Barcelona’s proactive approach is an exception. In most cities overwhelmed by tourists, it’s far more common for officials to deny that there’s a problem at all. Tourism ministers and travel industry professionals have historically been judged by the number of international arrivals—rather than, say, length of stay or contribution to the local economy—so the incentive has been simply to get people into the country, with little regard to how long they stick around or what they actually do. In an age of so-called “city breaks” and weekend trips, this no longer makes sense—even if twice as many people were to visit Munich or Bordeaux, they wouldn’t be adding any additional value (financial or otherwise) if they stayed half as long.

What’s more, oversaturated cities don’t appeal to people who would rather not spend their vacations dodging tour groups at museums and fighting for tables at overpriced restaurants. This suggests that class divides will be further exacerbated in places that ignore overtourism. As the very rich find new, secluded places to go, the majority of visitors will keep piling into the same popular cities, alongside entirely new demographics of tourists. And as the middle class booms in countries like China, millions of people will suddenly have the opportunity to travel abroad.

The term “overtourism” was coined in a research paper about overfishing in Indonesia more than a decade ago, and in many ways, those concerns about sustainability remain the same. What’s the best way for a city to cultivate tourism without wrecking life for residents, ruining subways and beaches, and pricing out people who have lived there for generations? Can city planners and economists anticipate when a destination is about to become unlivable, and do something about it in advance?

In the case of Lisbon, what’s happening now has been in the works for a while. After the 2011 debt crisis, the Portuguese government courted low-cost airlines, undertook a massive overseas digital-marketing strategy, and poured money into growing tourism. Seven years later, tourism is now the country’s largest employment sector, and the socialist government recently launched a program to lease 33 historical sites around the country into order to nudge visitors away from the hot spots of Lisbon and Porto. As was the case with Spain, public reactions to this have been mixed.

But Portugal, at least, has the benefit of being able to learn from the missteps of others. The next few years will reveal whether it decides to do so.