This paper provides an overview of the observations which support the presence of accretion in disks around pre-main sequence stars and of the characteristics of accretion inferred from their interpretation. The main properties of outbursting, high mass accretion disks, the FU Orionis objects, are summarized. Accretion in the Classical T Tauri stars is reviewed, including the magnetospheric flow and the accretion shock on the stellar surface. Accretion-driven wind properties are summarized. The structure and evolution of Classical T Tauri disks, powered mostly by irradiation from the central star, are discussed.