Abstract: This paper consists of three parts. The first deals with two-dimensional unsteady viscous separated flows. A number of separation criteria are briefly reviewed, and some progresses in identification of separation are discussed. The second involves the study of three-dimensional steady separated flows and the survey of the debatable question, whether a separation line is a limiting streamline or an envelope of the limiting strea-mlines. New achievements in this field are mentioned. The third elaborates the similarities between the equations of three-dimensional steady boundary layers and those of two-dimensional unsteady boundary layers. The question about analogous unsteady separation is discussed.