The capability of flow cytometry for rapid quantitative observations of individual cells has made it an important tool in plankton research since the mid-1980s. One indication of the impact of flow cytometry has been the appearance of special journal issues focused on applications in aquatic sciences (Cytometry, Yentsch and Horan 1989; Scientia Marina, Reckermann and Colijn 2000; Cytometry, Courties and Troussellier 2001). Since these publications, major advances have been made in in situ applications and in new approaches enabled by molecular and genomic techniques. In this chapter we provide a brief overview of the history of flow cytometry in plankton research, summarize basic measurement principles and review specific applications, and conclude with an assessment of some emerging areas and future prospects. We stress uses that depend on inherent fluorescence characteristics, with some related reference to uses involving fluorescent markers, stains or probes. Because a number of previous reviews and methodology guides are available (e.g., Chisholm et al. 1986; Legendre and Yentsch 1989; Yentsch and Horan 1989; Burkhill and Mantoura 1990; Olson et al. 1991, 1993; Troussellier et al. 1993; Collier and Campbell 1999; Collier 2000; Veldhuis and Kraay 2000; Vives-Rego et al. 2000; Campbell 2001; Legendre et al. 2001; Marie et al. 2005), our primary focus will be on recent and forward-looking approaches.