Flow cytometric (FCM) methodology represents a powerful analytical tool for screening and detecting abnormal and malignant cells, for subclassifying malignancies beyond conventional morphologic type and grade and for transcending light microscopic features by providing more biologically meaningful information. This paper reviews the historical background leading to the development of the FCM methodology and instrumentation that is presently being used for cytopathologic (and histopathologic) diagnosis, tumor subclassification and identification of aggressive cancers. Emphasis is placed on human carcinomas, with reference to hematologic malignancies when appropriate. Examples of FCM DNA content and cytokinetic techniques are described for the various cytologic sampling methods. The advantages, limitations and future prospects for the application of flow cytometric techniques in cytopathology are discussed.