Cytometry is the process of measuring the properties of individual cells. These properties may include gene or protein expression, chemical properties, deoxyribonucleic acid (DNA) content, and various cellular functions. The earliest methods of cytometry relied upon light microscopy for the classification and observation of cells and cellular components. Microscopy permitted direct visual observation of cells for the first time, leading to the classification of cells by morphology and insight into cellular functions. However, the time required for microscopic analysis constrains the number of samples or number of cells in each sample that can be examined. Therefore, the utility of microscopy for analysis of rare cells or in situations where sample throughput is a priority is limited. Flow cytometry was developed largely to improve upon these limitations.

Flow cytometry depends upon the ability to pass cells single file in a solution through the path of one or more laser beams. Flowing cells through the path of the laser permits the analysis of thousands of cells per second, but requires that the cells in the sample are in a single-cell, liquid suspension. Therefore, solid tissue samples require dissociation through mechanical or enzymatic processing before analysis. A wide variety of information about the cell can be determined depending upon how the cell interacts with the light from the laser. Detectors that measure the manner in which the cell scatters light can provide information about the size of the cell as well as complexity due to presence of intracellular granules and irregularities in the shape of the cell membrane. On the basis of light scatter properties alone, three distinct populations of leukocytes can be resolved in the peripheral blood: lymphocytes, monocytes and granulocytes. However, the power and flexibility of flow cytometry is fully realized when fluorescent probes that can be excited by the light from the laser are incorporated.