Flow cytometry (FC) is defined as a method for the qualitative and quantitative measurement of biological and physical properties of cells and other particles suspended within a high-velocity fluid stream and passing through a laser beam in a single file. This article is to familiarize the reader with the principles of FC, data analysis and interpretation, and most common applications (e.g., viability, apoptosis, cell cycle, cell physiology, immunophenotype, and protein production). Also described are some of the newest instruments that are available on the market.