At the beginning, phytoplankton primary productivity is defined and a terminology presented. Methods for estimating production are outlined and the most common techniques, such as oxygen evolution, carbon uptake, and fluorometry, are briefly described. Then the physiology of photosynthesis is emphasized, starting with the light harvesting mechanism followed by an outline of how carbon is reduced and acquired. Physiological responses through acclimation of algal assemblages and their time scales are presented. Problems of accurate measurement of respiration and photorespiration and their effects on the accurate estimation of photosynthetic rates are discussed. The response of phytoplankton productivity to environmental variability is highlighted via the analysis of photosynthesis–light response (PE curve) of phytoplankton. In addition, one of several mathematical descriptions of the PE response is given. Production in freshwaters is described in detail and various types of vertical profiles are compared. The daily, seasonal, and interannual variability of primary production is emphasized and finally primary production is assessed on a global scale. Primary production is here related to trophic categories of freshwaters and estimates of the production range for various climatic regions are summarized.