SYNOPSIS. A model relating environmental variability and population variability is proposed and discussed. The model strongly emphasizes the importance of the different consequences of erratic and regular environmental variability, which are discussed from both the theoretical and empirical viewpoints. Data on population variability for six species in three communities (temperate intertidal, subtropical intertidal, and oligotrophic lake benthic) are presented and discussed in the context of the model. In general, environmental variability and population variability are trongly correlated. The impact of erratic environmental variability is apparent in one community.