«The market seems to have come to the conclusion that cloud computing has a lot in common with obscenity–you may not be able to define it, but you’ll know it when you see it » (James Urquhart – The Wisdom of Clouds). Beyond this intrigouss definition, the presentation will introduce the general architecture and characteristics of Clouds and how Web applications are executed in Clouds. This talk will then focus on the difficulties encountered for testing applications deployed in Clouds and will present some methods which attempt to overcome these issues,under various assumptions.