Some theoretical results obtained in a study unsteady flows are presented. The field of time-dependent hydro-dynamic phenomena is approached from a general point of view. A few special flows are discussed which emphasize the diversity of unsteady flow problems. An analysis of local and convective acceleration is presented in detail. A dimensionless ratio of these accelerations is introduced. With the magnitude of this ratio, the unsteadiness of the flow can be described and characterized. The measure of unsteadiness is generlly space and time dependent. A short discussion of accelerationless flows is included.