The study of cephalopod populations currently lacks the means to define populations adequately and to resolve basic systematic confusions. Quantitative data are usually only available from indirect sources such as commercial fisheries and from estimates of consumption by higher predators. Despite these methodological difficulties it is clear that cephalopods comprise a major component of biomass globally, throughout all fully marine habitats. Life-cycle characteristics common to the coleoids - early and/or semelparous breeding, rapid growth, short lifespan, little overlap of generations, vulnerability to predation and environmental variables - result in wide inter-annual fluctuations in abundance. Most of the pelagic forms also undertake large- or meso-scale migrations which, coupled to shifting patterns of oceanographic variables, contribute to the unpredictability of distribution and density associated with many cephalopod species. Temporal and spatial patterns of breeding, seasonality, growth, recruitment and mortality are clearly evident in most of the better-studied species. But exceptions to pattern (e.g. variable growth rates, extended breeding, complex recruitment) also seem to be important intrinsic characteristics. Levels of genetic variation in cephalopods are relatively low, and their population dynamics appear to be influenced principally by phenotypic plasticity in response to environmental variability. In such universally short-lived species the maintenance of this diversity balances the risks of mortality factors combining at any one time to cause periodic local extinction. The extent and scale of the interactions between cephalopod populations and other trophic levels suggests that major ecological perturbations such as environmental shifts, or imposed effects such as commercial fishing, whether directed at cephalopods or other species, are likely to have an impact on their populations. As short-lived species with high turnover of generations, plastic growth and reproductive characteristics, high mobility and catholic predatory habits, they are always poised to respond to changed balances in their environment. Studies on cephalopod populations have expanded considerably in numbers and scope in the last 25 years, driven by increased interest in and recognition of their roles in the marine ecology, as well as their increasing value as globally exploited resources. Despite these recent advances, the information and concepts arising from their study is only slowly entering mainstream biological thought and becoming accommodated in broad-scale models of the marine ecosystem.