Flow cytometry is an analytical technique in which optical measurements are made of cells or other biological particles as the cells or particles flow, ideally in single file, in a fluid stream past one or more optical measurement stations. Modern optical flow cytometers typically measure light scattered at small (1-5°) and large (15°-135°) angles to an illuminating laser beam, and fluorescence emitted in three or more discrete spectral bands; the most complex instruments employ three or four spatially separated illuminating beams at different wavelengths and can measure twelve fluorescence signals from each cell or particle analyzed.

Flow cytometry was developed in the 1960's to speed up and automate microspectrophotometry and microfluorometry, which had previously been microscope-based, in order to facilitate development of observer-independent clinical tests which could replace the Papanicolaou smear and the differential leukocyte count. At that time, high-resolution scanning was an extremely slow process; high-intensity sources were restricted to arc lamps, and memory in which images could be stored was expensive.