Abstract: Chromium is a heavy metal of commercial importance, thus significant amounts are released in wastewaters. Chromium in wastewaters and in the aquatic environment is primarily encountered in oxidation stages +3 (Cr (III) ) and +6 (Cr (VI) ). Recent publications suggest that Cr (VI) compounds are more toxic than Cr (III) ones, while Cr (III) has been identified as trace element, at least for complex organisms. With respect to chromium species mobility, Cr (VI) can cross cellular membranes, which then may be oxidized to Cr (III) and react with intracellular biomolecules. Clear conclusions cannot be derived about the critical chromium concentrations that affect activated sludge growth, as the latter is a function of a number of factors. Broadly, may be supported that activated sludge growth is stimulated at Cr (III) concentrations up to 15 mg L −1 , above which is inhibited, with lethal doses lying above 160 mg Cr (III) L −1 . On the other hand, literature data on Cr (VI) effects on activated sludge are even more controversial. A number of reports support that Cr (VI) is toxic to activated sludge at concentrations above 5 mg L −1 , while others report growth stimulation at concentrations up to 25 mg L −1 . However, all reports agree that Cr (VI) is definitely an activated sludge growth inhibitor at higher concentrations, while 80 mg Cr (VI) L −1 have been identified as lethal dose. A number of factors have been identified to influence chromium toxicity on activated sludge, such as, pH, biomass concentration, presence of organic substances or other heavy metals, acclimation process, exposure time, etc. Naturally, the presence of chromium species in wastewaters may affect the performance of wastewater treatment plants often causing malfunctions, particularly for industrial wastewaters containing relatively high chromium concentrations. The present work reviews in a critical way the published literature on chromium effects on activated sludge, and on the operation of wastewater treatment plants.