Flow cytometry theory overview Flow cytometry is a technique used to count and examine cells or subcellular components by suspending them in a flowing saline stream and passing them through an electronic device, called a flow cytometer. It is a very useful tool for sorting and counting cells, for example, differentiating between abnormal cells and normal cells. Fluorescence-activated cell sorting (FACS) is a specialized type of flow cytometry. It is commonly used in sorting cells from a heterogeneous mixture into separate containers based on the recording of light scattering and fluorescent signals that cells emit. . Schematic steps in FACS .