This collection aims to make an inventory of the publications using flow cytometry in the field of oceanography and microbiology. The definition of flow cytometry is easily deduced from the etymology of its name: measurement (-metry) of the optical properties of cells (cyto-) transported by a vector liquid (flux) to a light source excitation . High-speed analysis performed at the single cell level of free particles (cells) is used to study the structure and properties of the microbial assemblages. Flow cytometry has quickly emerged as a technique of choice by allowing a qualitative and quantitative analysis of individual cells at a frequency of up to several tens of thousands of cells per second. Advances in technology, along with progress in fluorescent markers and molecular biology promise new opportunities for microbiologists in the near future to increase their knowledge on aquatic bacteria, phytoplankton, small protozoa and viruses.