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This review discusses critically how seasonal changes might affect the community composition and dynamics of activated sludge wastewater treatment plants, and examines the factors thought more generally to control microbial community assembly, including the role of taxa–time relationships and stochastic and deterministic influences. The review also questions the differences in protocols used in these studies, which make any subsequent attempts at data comparisons problematic. These include bacterial DNA extraction and PCR methodologies, 16S rRNA sequencing and especially its depth, and subsequent statistical analyses of the data, which together often fail to reveal seasonal dynamic community shifts. Suggestions are given as to how experimental protocols need to be improved and standardized, and especially the requirement to examine bacterial populations at the species level. This review looks critically at what is known currently about seasonal influences on key members of this community, including viruses, the bacteria responsible for nitrogen and phosphorus removal and those causing bulking and foaming. The data show many of these species exhibit replicative seasonal abundances over several years, but not under all conditions, illustrating how complex these community dynamics are. Fungal and protozoal/metazoal seasonal community dynamics, less studied, are also discussed. The current data suggest that seasonal temperature fluctuations are responsible for most of the seasonal community dynamics by selectively favouring or otherwise individual populations. However, more longer term studies carried out under much stricter controlled conditions are required. [ABSTRACT FROM AUTHOR]