In this review, we describe the principles of flow cytometry Hydrobiology areas of study is intended to promote flow cytometry. Flow cytometry was developed in the 1950s, today is also used in different studies. Using the principle of light scattering and absorption, flow cytometry can be used, especially algae. In speed, accuracy, cost and convenience in terms of unique opportunities, explained the principle of flow cytometry study. The review primarily focuses on how flow cytometry is used in hydrology. Another aim of the review is to encourage re-searchers to use this application more in their studies by describing flow cytometry in its gen-eral guidelines.