In this chapter I first present the pre-main sequence stars around which accretion disks are observed and summarise the main observed characteristics of the classical T Tauri and Herbig Ae/Be stars. I then review the theoretical and empirical reasons that lead many scientists to conclude that the objects of this class are surrounded by accretion disk. Finally I review the basic characteristics of the disks of these stars, and expose the observations that lead us to derive them.