Flow cytometry is a method to conduct a multiparameter analysis of cells suspended in liquid and passing through a laser beam. As cells pass through the flow chamber they are hit with the laser light beam which is scattered in different directions and recorded as forward light scatter and side light scatter. In this book, the authors discuss the principles, methodology and applications of flow cytometry. Topics include flow cytometry and epifluorescence analyses of freshwater bacterio- and virioplankton communities; application of flow cytometry in phenotyping, analysis and functional characterisation of T regulatory and immune suppressor cells; multicolor panel design strategies in rare event analysis of dendritic cells and regulatory T cells; advanced characterisation of individual particle morphology light-scattering flow cytometry; and application of flow cytometry in environmental science.