Argentina features a large variety of freshwater phytoplankton habitats. They include rivers, large, deep, glacier-fed oligotrophic lakes in the Andean Cordillera (Patagonia) including those straddling the border to Chile, as well as shallow lakes and reservoirs, distributed along this territory. The most distinctive hydrological feature of Argentina, however, is the La Plata watershed. The broad valleys of the Paraguay, Paraná and Uruguay rivers drain nearly one-quarter of the surface area of South America and carry a similar fraction of the continents fluvial discharge into the Atlantic Ocean. Intensive studies have been carried out on this system of rivers from the headwaters in the Chaco and the Mato Grosso of Brasil to the delta near Buenos Aires. They also include the flood plains and shallow lakes of the Iberá and Entre Rios regions and are reflected in the subdivision of chapters in this book. Many of the previously published observations already have a strong comparative element. The intent of this volume is to provide an overview of the research on phytoplankton assemblages in these waters. The overview is presented largely on a habitat-by-habitat basis but individual overviews also discuss generic aspects of the physiology, energy harvesting and production of phytoplankton, including mixotrophy. Some practical issues related to the supply of drinking water and the occurrence of harmful algae are also addressed; even the initiation of paleolimnology in Argentina is described. While much research still remains to be done in all areas of a territory as large as Argentina, this work is a valuable contribution to the knowledge of freshwater phytoplankton assemblages of the main aquatic ecosystems of the country. Moreover, the chapters add to and update the information available on the biodiversity of freshwater phytoplankton worldwide, chiefly because the Argentinian freshwater ecosystems studied and discussed in the chapters of this volume are so highly diverse.