We know a lot about them. Or, at least, we think we do.
They spend hours every day on their phones. They want convenience, technology and style, but for the right price.
Would it even be possible to write about them without mentioning avocados?
But there are no two ways about it - millennials and Gen Z have already inherited the earth. And as their influence and spending power grows, their travel desires - how they do it, why they do it, and what they want from it - are having a huge impact on the hotel industry.
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• Pictures first: Millennials won't book holidays without seeing it on social media first
They do all their research online; they take photographs all the time; they'd rather spend their money on getting out into the world than heading into a mall. All of which makes them natural-born explorers with a very clear idea of what they want from a hotel.
And what is that?
They want high-end
Whether it's a fashionable micro-hotel in the middle of New York City, a riad in Essaouira, or sheepskin-clad yurt in the Australian outback, a high-end experience is now a top priority, even if the budget is low.
This can play out in any number of ways. Think interiors (perhaps pastel tiled bathrooms, naughty wallpaper or social areas decked out in contemporary local art), service ("Fancy a cocktail? We'll send the mixologist right up") or facilities (a DJ in reception, gym classes in the basement, a mini-golf course on the roof).
Take Moxy Hotels' collection of almost 50 international hotels, which deliver impeccably stylish hotel rooms that are compact but practically perfect in layout and functionality. What this hip chain lacks in reception desks and in-room alarm clocks is countered with reception areas that transform into bars, hushed co-working spaces, rooftop bars and fashionable restaurants.
But they also want authenticity
It's no longer about accumulating products, it's about accumulating "real" experiences.
Next-gen travellers don't want to lie in a resort, sheltered from the culture of the country they are visiting - they want to be out in it, dining at a guide's home, eating at the night market or at the best food truck in the city, engaging with the locals.
Within the hotels' landscape, they want to know that their experience is unique, so gone are the days of bland cookie-cutter hotel rooms. The next-gen want to sleep in a room that feels personal - a guitar in the corner, a welcoming do-it-yourself cocktail kit, a personalised message on a chalkboard, local craft beer in the minibar, and local artwork on the walls.
They want convenience, and flawless technology
There's no time to check in face to face. Instead, download the hotel app, push a button to let them know you've arrived, and a drink will be waiting at the bar for you.
There's no key card necessary - your phone can unlock your door once you're ready to go up.
There's no alarm clock or room service, but there'll be Netflix on the television and high-speed codeless internet to stream whatever you like.
If you really must have room service, at EMC2 in Chicago, just use your in-room Alexa and she'll send a robot right up.
And you bet it's got to look good
Pictures or it didn't happen, right? Every millennial and Gen Zer is a documentarian - filming, photographing and recording every step of the way.
And the hotel industry is ready for them, savvy to the power of a social post, each nook and cranny ready to be snapped.
Whether it's seed-topped smoothies, rooftop rope swings with views over the city, exposed copper plumbing, or neon signs lighting up exposed brick walls - every inch of any Gen Z and millennial-friendly hotel will be prepped and ready for that million-dollar shot.
The world's most millennial hotels
W Hotels, worldwide
Futuristic interiors, rooftop pools, ideal locations (think city centres or shoreside), W Hotels has the next-gen market enamoured with more than 50 hotels across 25 countries.
From the rooftop basketball court at W South Beach, Miami, to the London mixologist who brings his portable bar to your room, W Hotels has led the way in catering to a new breed of traveller.
Moxy Hotels, worldwide
Stretching from Tbilisi to Tokyo, the Moxy empire - owned by Marriott - is nearly 50 strong, with dozens more in the pipeline. The focus is high style, low price. Rooms are small but perfectly formed, and public spaces are maximised and designed to encourage interaction, from bars and brasseries to art installations and gym classes.
There are no check-in desks, alarm clocks or room service, but you're likely to find rooftop Twister, Netflix in your room, electro-pop blazing in the foyer and a rubber duckie waiting on your pillow.
I challenge you to find a corner of El Fenn that you could resist photographing. Owned by Vanessa Branson, sister of Richard, El Fenn is a riad experience in Marrakech that has three pools, a yoga lawn and more pink and green interiors than your followers can possibly stomach.
Ace Hotels, US and UK
With eight hotels in the US, one in London's Shoreditch, and two openings due this year, Ace's star is in the ascendancy. The crowd is young and cool, and the rooms feel like home, but cooler - vintage furniture, band posters, the occasional guitar or record player. Facilities vary from co-working spaces to DJ booths.