As today Informatics is more and more (driven) eaten by its applications, it becomes more and more important to know what can be done and what cannot be done. For a long time now, this is well known in sequential computing. But today the world becomes more and more distributed, and consequently more and more applications are distributed. As a result, it becomes important, or even crucial, to understand what is distributed computing and which are its power and its limits. This article is a step in this direction when looking from an agreement-oriented fault-tolerance point of view.