This article reviews historical development of our understanding of protoplanetary disks. Pro- toplatary disks have been studied by various types of observations ranging from optical through infrared to radio emissions. The progress of our understanding deeply owes technological ad- vances in the observations. This article reviews various methods to identify the protoplanetary disks. They include infrared color (spectral energy distribution), the Doppler shift due to rotation, imaging, and indirect evidences such as jets and outflows. It also lists up surveys useful for the study of protoplanetary disks. After several recent topics interesting the author are introduced, it is concluded with a short summary.