Flip through a travel brochure, and you're likely to see pictures of sun and sand in Southeast Asia, of luxury lodges in the Serengeti, of scuba diving in the bejewelled coral reefs of the Caribbean.
What the brochure won't reveal, though, is tourism's dirty little secret: the environmental cost of your trip.
That beach in Thailand may once have housed precious mangroves, which were ripped up to make way for your hotel.
To provide you with a piping-hot shower and tempting meal after a safari, your Serengeti lodge may be tapping into precious water supplies, dumping waste in exchange and paying your waiter just a dollar a day.
And to get you to the Caribbean, your plane will spew out tonnes of carbon pollution, thus stoking the global warming which is killing the very corals you want to enjoy.
This is where eco-tourism comes in.
One of the fastest-expanding and well-heeled sectors of the travel industry, eco-tourism aims at serving the growing numbers of people who want to see exotic sights, rare wildlife and remote cultures, but feel guilty about the footprint they will leave.
"About 70 million people each year travel to places with fragile eco-systems and cultures under what you might call eco-tourism," Tensie Whelan, executive director of the green group Rainforest Alliance, told AFP.
According to the Washington-based group The International Ecotourism Society (TIES), global eco-tourism has been expanding at rates of between 20 and 34 percent a year since 1990 -- and in 2004, the business grew three times faster than the tourism sector as a whole.
The typical eco-tourist is likely to be an experienced traveller aged 40-plus with higher education and in the top earning brackets, says TIES.
The Worldwatch Institute, a US green group, defines eco-tourism as "responsible travel to natural areas that conserves the environment and improves the welfare of local people."
Under this broad umbrella comes a wide range of activities. They can operate on a scale that ranges from the personal to the mass market, and their green benefit is highly variable.
Projects include tiny groups of people who accompany conservationists into the Amazon to document wildlife or who are given a close-up look at chimpanzees in ancient forests in Africa.
At the other end of the numbers scale, South Africa's well-run National Parks plough fees from visitors into sustaining and policing the reserve.
Energy efficiency, water conservation, transport and renewable resources are big features in eco-tourism. Asking environmentally-sensitive guests to re-use their towels is not enough.
To win credibility with this upscale, demanding slice of the market, hotels and lodges have to offer such things low-flush toilets, bicycle hire, solar-powered water heating and solar-powered electricity, intelligent lighting or air conditioning panels.
Many pledge donations to preserve the local nature reserve or promise to help the local community with good jobs or locally-sourced materials.
Another inducement in eco-tourism is carbon "offsets" to compensate for the pollution of the client's holiday. "Offsets" are schemes by which a polluter buys into a project elsewhere that will compensate for the amount of carbon dioxide (CO2) resulting from his trip.
For instance, a 10,000-kilometre (6,000-mile) flight would emit around 1.5 tonnes of CO2. An "offset" to counterbalance this, such as planting trees or investing in cleaner energy in the Third World, would cost around 27 dollars at present prices.
The rise of eco-tourism has in turn spurred fears that it can be exploited cynically as greenwash, masking projects that are environmentally destructive and culturally erosive.
Tricia Barnett of the British campaign group Tourism Concern complains there are more than 400 certification schemes for eco-tourism around the world, a good many of which are simply "good marketing."
"You can go to a tented encampment in Zambia or somewhere and find that you have porcelaine toilets fitted in. Those are the people who say their projects are eco-tourism, when it's really about making a niche product."
Neel Inamdar, an expert with the US group Conservation International, agrees that the issue of classification "is a major problem," as there is no universally accepted definition of sustainable tourism.
From next year, though, a panel comprising non-profit groups, UN agencies and conservationists will take the first steps towards establishing a global label.
Whelan, whose group, the Rainforest Alliance, has joined with Conservation International and others in the certification scheme, says travel cannot be ignored and the only option is to coax it into greenness.
"There's always going to be mass tourism," she said. "The question is: do you allow cheap mass tourism that's going to be very destructive, or do you try to change that mass tourism?"