Cephalopods are voracious, versatile predators. They generally have a short life span and a single spawning event followed by death. Populations are subject to dramatic fluctuations and their impact on prey populations is equally variable. The prehensile arms and tentacles of cephalopods, coupled with a highly evolved sensory system, allow them to occupy a broad trophic niche and migrations enable populations to exploit the temporal and spatial variability of production systems and populations of prey. Shoaling is a common behavioural feature of many species which facilitates prey capture and contributes to the impact of cephalopods on prey populations. Research on cephalopod stomach contents is hampered because the beak is used to bite the prey into small pieces so hard parts, which are usually needed for identification of prey species, are often rejected causing potential bias in estimation of diet. Cephalopods may also feed unnaturally in the presence of sampling gear. Despite these problems there is a growing body of data on cephalopod predation collected using direct observations, conventional visual analysis of stomach contents and serological methods. Most species feed on small crustaceans as juveniles and shift the diet to larger fish and other cephalopods during growth. This shift is accompanied by ontogenetic changes in the allometry of the brachial crown. There is increasing evidence that myctophid fishes are an important food resource for oceanic squid. The diet and stock size of some commercially exploited squid populations is sufficiently well known to quantify the impact of a single generation on the prey community. Where there is predation on commercial stocks of fish and crustaceans, the effect of cephalopod feeding on recruitment may be significant. Cephalopods are trophic opportunists in marine food webs from polar to equatorial seas.