Since the rediscovery of picophytoplankton (PPP) in 1978 -1979, the information and method of studying of these living organisms have improving quickly. This review collects the most important methods for studding these tiny planktonic which play a significant role in all aquatic ecosystems. In addition to, give an over view on the results obtained on the contributions, abundant and role of these groups in different habitats and they correlation with physical, chemical and environmental parameters, such as temperature, light intensity and quality, salinity, pH, trophic state, and grazing. The development of gene sequencing, the molecular approach and phylogenetic analysis give an advanced direction to taxonomy of PPP and their relation and interactions to environmental factor, therefore the review shows the new results of this topic.