The one thing everybody can agree on, though, is that it's huge.
There are two things which have influenced the growth of tourism.
They are firstly -- social factors, and secondly -- technology and the way it's developed.
Let's consider the social factors first of all.
Demand for tourism is determined mainly by the amount of wealth a country has, which is why countries such as Japan, Australia, the USA and western European countries have contributed most in terms of tourist numbers in the past.
However, growing wealth in developing countries will mean that demand for holidays abroad will take off there in the near future, boosting tourism enormously.
That said, the majority of tourists are still from what are called the developed nations.
However, studies show that their number will not rise much further in the next few decades because their populations are fairly stable.
As a result, there'll be a growth in the number of retired people who'll have more time on their hands.
This will influece the kind of tourism wanted: fewer skiing holidays will be required, but there'll be an increase in the number of people wanting to visit art galleries.
