The Caspian Sea ecosystem, with its high level of endemic taxa has suffered from anthropogenic pollution, changes in the quantity of nutrients, and the effects of invasive species since the 1980s. The change in phytoplankton community was undertaken in the southwestern Caspian Sea between 1996 and 2010. An increase in phytoplankton abundance was observed after 2000, and has continued until 2010. Except in 2001 and 2002 diatoms predominated throughout the study. A decrease in silicate levels and an increase in water temperature, salinity and stratification were the main factors causing a bloom of dinoflagellate Prorocentrum cordatum and cyanophyte Oscillatoria sp. during 2001 and 2002. There were major changes in environmental parameters, increases in nutrients concentrations (DIP & DIN) were more than two-folds after 2000 as compared in 1996?1997. Numerous causes might have contributed to the changes in the phytoplankton community of the southwestern Caspian Sea, especially the severe environmental degradation during the last two decades. It is hoped that this book will offer an important source for students and younger scientists who are working in marine ecology.