Flow cytometry, a concept originated back in 1934 by Moldavan (Moldavan 1934), has evolved to become an indispensable bioanalysis tool, enabling researchers to study and characterize the physical (cell size, shape, and granularity) and biochemical (DNA content, cell cycle distribution, and viability) properties of cells i n a h ighly quantitative manner. Besides its applications i n basic research (e.g., i mmunology, and cell and molecular biology), this technology has allowed hematologists to de tect and monitor the progression of diseases such as acute myeloid leukemia (AML) (Jennings and Foon 1997b) and AIDS (Shapiro 2003). A state-of-the-art ow cytometer (also known as a uorescence-activated cell sorter, or FACS) can interrogate and sort cells with a throughput of tens of thousands of cells per second, making possible rare-event studies, such as the identi cation of bacterial cells (Casamayor et a l. 2007) or the isolation of stem cells (Gratama et al. 1998). Currently, more than 30,000 ow cytometers have been employed in various research institutions and hospitals (Herzengberg et al. 2002), and this number has been g rowing a s re cent te chnological adv ances a llow ow c ytometers to b ecome more s ophisticated (multicolor detection and sorting capabilities), cost e ective (<$50,000 for basic models), and less bulky. While the proliferation of these machines has been impressive, their basic operational principles have remained the same, and there is still room for paradigm-shi ing improvements-especially with regard to the intrinsic limitations of the system (e.g., serial inspection and pressure-velocity relationship versus cell viability (Engh 2000)). In this chapter, we brie y discuss cytometric principles and system components of a ow cytometer as well as its applications in research, and clinical and biotechnological settings. Additionally, a survey regarding the performances of some of the state-of-the-art ow cytometers developed by di erent companies will be presented.