Flow cytometry is literally measuring cells while moving in a liquid. More specifically, a suspension of single cells is labeled with one or several fluorescent labels. In the machine, the cells are constrained into single file. These cells pass through one or more laser beams to excite the fluorescent labels. The light emitted from the fluorescent labels is collected, separated, measured, and the resulting data transmitted to the computer controlling the instrument. In addition, narrow-angle and 90? light scatter from the laser beam are measured and the data are also sent to the computer. All of these values are recorded as correlated measurements for each cell separately. The data are displayed in the computer as single-parameter histograms or twoparameter plots. The software allows populations to be identified and specific subpopulations selected for further analysis. The number and fraction of cells in specific populations can be quantitated. In addition, the amount of the fluorescent label can be calibrated, and by extension, the amount of the ligand for the label can be calculated, if needed. The patterns of expression of specific cellular proteins or changes in numbers of cells in specific populations are used to contribute to diagnosis of the patient.