Whether they're spoken about openly, or talked about in hushed tones, there are corners of almost every major city where the sex trade is thriving.
From Nevada to Amsterdam, Paris, and Bangkok, red-light districts have occupied a place in the minds of artists, writers, self-professed bohemians, moralists, ethicists, and travellers for centuries.
Of course, a tangle of thorny dilemmas need to be untied before you consider adding the world's most notorious red-light districts to your itinerary. First and foremost, you'll want to consider what it means to partake in a spectacle that quite often exploits already marginalised communities of women, LGBTQ people, the economically disadvantaged, and people of colour. It's important to keep in mind that many of those who participate in the sex trade are not doing so consensually.
There's also the question of legality. While cities like Amsterdam have notoriously liberal laws that, ostensibly, make entering the sex trade a choice that's regulated (and therefore sanitised and destigmatised), in other parts of the world, it's an outright crime.
This is to say nothing of the involvement of sex work in exacerbating already existing cycles of poverty and violence in regions where tourism has already contributed to single-sector economies.
Now, with all that in mind, here are a few of the world's most infamous red-light districts, ranging from outright sexual free-for-alls to sanitised tolerance zones and violent neighbourhoods where tourists should never be seen.
Consider yourself warned.
1. PATPONG MARKET, BANGKOK, THAILAND
With its sky-high temperatures and fast pace of life, Bangkok is essentially primed for all things steamy and sordid. So it's not surprising that the city's notoriously laid-back attitudes toward sex work - it's not exactly legal, but not illegal either - have made Bangkok a hub for the trade in Southeast Asia.
And while those facts certainly means that you're in for an experience walking the streets around Patpong Market, Soi Nana, and Soi Cowboy, it can feel more than a little insidious, and concerns about human trafficking should not be underestimated.
A tour of Patpong Market, past the mouth-watering street food stands and tables full of cheap T-shirts, is an assault on senses and sensibilities. You'll hear hawkers selling everything from ping-pong shows to live sex, and most - if not all - of the participants can be had for a price. However, if you talk to most people who've witnessed these escapades, the effect is far from arousing.
For those who do choose to step into the bars and clubs in these parts of town - particularly in Patpong - expect strictly enforced (and exorbitantly priced) drink minimums, aggressive management, and high-pressure sales tactics.
2. DEWALLEN, AMSTERDAM, THE NETHERLANDS
Amsterdam may conjure many images in the minds of travellers: charming canals, bright orange Olympic speed-skating outfits, quaint row houses, the smell of weed, and Dutch pancakes all come to mind.
Of course, it also has one of the world's most well-known red-light districts - and it's a major draw on the city's tourist circuit.
In Amsterdam, sex workers occupy windowed cabins along the streets of the De Wallen, a gentrifying part of town that's home to students, tourists, and sex workers alike.
Laws since 2000 have cleaned up what was once a part of town that was plagued by social ills. These days, pimping is illegal, brothels must be licensed, and everyone involved - from the client to the worker - pay taxes. The city even has a union for sex workers.
Additionally, organisations like the Prostitution Information Center offer pro-sex and pro-legalisation platforms for sex workers in the city (as well as tours of the De Wallen with former sex workers). The aim is to destigmatise those in the industry while empowering workers who choose to engage in this line of employment.
Even so, this isn't to say that it's all rosy in Amsterdam's sex trade, despite the hazy pink glow cast over the De Wallen from its iconic red lights. While the window-bound form of sex work is heavily regulated, ethical concerns still flourish about other forms of street-based sex work, and despite numerous arrests and crackdowns, human trafficking continues to be present in the city.
3. PIGALLE, PARIS, FRANCE
Ah, Paris - it's a city for romance and love. All around, doe-eyed couples ogle sights like the Eiffel Tower, the gilt halls of Versailles, and the collection at the Louvre.
Perhaps it makes sense that the sexier side of life can be found here as well - and even though sex work is illegal in Paris, officials generally turn a blind eye to the industry. In fact, if you're heading up to the stunning Basilica of Sacre Coeur, or snapping a selfie with the Moulin Rouge, you'll be traipsing through what is still Paris' most active red-light district.
Sex shops, porn stores, strip clubs, and peep shows are abundant throughout Pigalle, and street workers ply the busier areas as well. You should note that petty crime isn't unheard of in this part of town, and soliciting sex workers is illegal in France.
Pigalle is also home to lots of rowdy bars and clubs, and when alcohol-fuelled belligerence gets added to the mix, the vibe can escalate quickly. However, like any good metropolis worth its weight in capitalist salt, the area is rapidly gentrifying, particular in what's known as South Pigalle - a haven for the hipster elite to spend their money of bespoke cocktails, one-off hoodies, and tiny plates of food.
4. KABUKICHO, TOKYO, JAPAN
The host and hostess clubs of Kabukicho are the stuff of legend. Right smack in the middle of one of the busiest tourist quarters of Tokyo - the Shinjuku district - sits this sex-packed part of town.
Here, bars staffed exclusively by pretty young men and women host visitors, and rates coincide with the club's reputation. In most of these establishments, the men and women are paid to dote on customers, but services remain fairly tame (conversation and flirting).
However, bars where sex is for sale are certainly on nearly every block in this part of Tokyo as well. It's also worth noting that management can get extremely aggressive about how much a customer owes for drinks during any visit - and that sometimes doesn't match the price quoted upon entering the establishment.
Foreigners are sometimes excluded from these venues. And in case you thought that well-mannered Japan would have a spick-and-span sex trade, think again. Sex work is technically illegal (though there are ways around these laws), and organised crime plays a major part in the daily goings on in Kabukicho.
5. PATONG, PHUKET, THAILAND
Plenty of travellers land in Phuket with visions of palm-lined beaches, the turquoise Andaman Sea, and pristine beaches. And all of that can certainly be found across this tropical island. However, there are pockets where anything but basking in the sun is on the mind - particularly in Patong.
The town is Phuket's main party hub, and rowdy nightclubs and bars pound with live music until late. Much like Bangkok's red-light districts, you'll also find all manner of for-pay sex, sex shows, and anything you can imagine along with it.
The heart of the action is clustered along Bangla Road, which is where some of the more notorious bars and clubs can be found, along with sex workers on seemingly every sidewalk.
Keep your wits about you, as you would in Bangkok, as even entering one of the go-go bars can come with pretty steep fees for drinks, often aggressive management, and more-than-pushy demands.
6. CALLE PEDRO CLISANTE, SOSUA, DOMINICAN REPUBLIC
To be fair, this isn't an outright red-light district - you won't see sex shows and porn shops lined up side-by-side. Instead, for visitors to Sosua, Pedro Clisante is the town's night-life hub.
Restaurants and bars set up shop along the street, with patrons spilling out all around. Within the mix, of course, are the sex workers that are a fixture in many of the Dominican Republic's beachside towns. Some travellers find them aggressive, others as simply a part of the local scene, and others take little notice. But they are omnipresent, particularly in the areas around the biggest nightclubs.
Like many sunbelt nations where economic inequality is rampant, issues persist here about exploitation, organised crime, and the vast network of social ills that have caused sex work to - legally - flourish. It certainly doesn't help that plenty of western tourists flock to Sosua flush with expendable cash and a disregard for consequences.
7. SCHIPPERSKWARTIER, ANTWERP, BELGIUM
Tolerance zones have flourished in cities across Northern Europe as attitudes toward everything from drug addiction to sex work have grown more progressive. In turn, the involvement of organised crime, rates of disease, and other once-dangerous elements of society's formerly illicit corners have been folded into the urban fabric.
In Antwerp's Schipperskwartier, this translates into an almost impeccably clean red-light district that blends Amsterdam's legalised approach with something like a mini-mall of sex.
With Villa Tinto at its core, this three-block section of town draws those looking for sex and those just looking to check out the scene. This part of town is also a go-to destination for clubs and night-life.
Mention Nevada and the bright lights, easy money, and unrestrained hedonism of Las Vegas likely springs to mind. Some might also think of the oft-repeated assertion that it's home to legalised prostitution.
That's not entirely true, and the laws governing sex work vary from county to county. In fact, Cook County (which is home to Las Vegas) does not allow legalised sex work to take place within its borders. As such, the red-light status of Nevada is a bit more diffuse.
Most of the legal brothels in Nevada are quite far from its major urban centres (according to Nevada law, the industry is only legal in counties with a population of less than 700,000). The government also requires regular STD testing and mandatory condom usage.
9. COPACABANA, IPANEMA AND VILA MIMOSA, RIO DE JANEIRO, BRAZIL
Much like Sosua in the Dominican Republic, the lack of restrictions on sex work in Brazil mean that while the trade flourishes, it's a bit more integrated into the day-to-day fabric of the city at large.
As such, there aren't parts of Rio de Janeiro that are jam-packed with sex shops and porn theatres. Instead, massive brothels can be found in many of the major beachside neighbourhoods, including Copacabana and Ipanema.
Copacabana, with its faded glamour, has a seedier vibe, and just to its south sits one of Latin America's most infamous brothels, Centaurus. This mega-brothel is built for high rollers and has drawn plenty of celebrities, though there are other go-to brothels in Ipanema, like Monte Carlo.
If you feel like taking your personal safety for a risky spin, Vila Mimosa is the true red-light district in town, where storefronts operate as cover businesses for a vast network of sex workers, pimps, and the related hangers-on in this industry. However, you definitely shouldn't attempt visiting this neighbourhood on a casual jaunt - it's unsafe for anyone but seasoned locals and has a violent reputation.
In any case, as with most areas where income gaps and inequality persist, the agency of those involved in the trade is not always clear, and many venues charge exorbitantly high rental rates of their workers, keeping them entrenched in poverty (and debt) while offering the veneer of fast cash.
10. REEPERBAHN, HAMBURG, GERMANY
Amsterdam's De Wallen gets the lion's share of attention when it comes to European red-light districts, but Reeperbahn in Hamburg is equally notorious. The area is packed with nightclubs and bars, making it a go-to spot for travellers and locals who want to let off some steam on the weekends.
It's also home to Herbertstrasse, where women and those under 18 are strictly forbidden.
In fact, the street itself is gated off to help enforce this policy.
Otherwise, the district is a melange of sex shops, porn stores, and the like, all mixed in with people in search of good times.
Sex work is legal in Germany, so there are regulations surrounding the endeavour that, at least on the surface, keep things feeling a bit less tawdry than in other parts of the world. Even so, don't come here expecting the warm and fuzzies.