Most animals grow substantially during their lifetime and change in competitive ability, predatory capacity and their susceptibility to predation as they grow. This thesis addresses the implications of this on regulation and dynamics within populations as well as between population interactions. In size-structured populations either reproduction or maturation may be more limiting. If juveniles are competitively superior, the competitive bottleneck will be in the adults and reproduction will be limiting. Mortality will in this case result in overcompensation in juvenile biomass through increased reproduction. Compensation in biomass was demonstrated in Daphnia pulex populations subjected to size-independent mortality, where juvenile biomass did not decrease when a substantial harvest was imposed due to increase per capita fecundity. This supported that juveniles were superior competitors and that population cycles seen in Daphnia are juvenile-driven. Compensatory responses in biomass may lead to that predators facilitate eachothers existence by feeding on a common prey, a phenomenon coined emergent facilitation. In an experimental test of the mechanism behind emergent facilitation it was demonstrated that the invertebrate predator Bythotrephes longimanus was favoured by thinning of its prey Holopedium gibberum. The thinning mimicked fish predation and targeted large individuals while Bythotrephes preferrs small prey. Size dependent predation also occurs within populations, i.e. cannibalism, were large individuals feed on smaller conspecifics. Two populations of the common guppy (Poecilia reticulata) originating from different environments were demonstrated to differ in cannibalistic degree. Cannibalism was also affected by the presence of refuges and females and juveniles from one population were better adapted to structural complexity than the other. The effects of these differences in cannibalism on population regulation and dynamics were studied in long term population experiments. Both populations were regulated by cannibalism in the absence of refuges, and displayed cannibal-driven cycles with suppression of recruitment and high population variability. The presence of refuges decreased density dependence and population variability and harvesting of large females in the absence of refuges led to population extinctions in the more cannibalistic population. The less cannibalistic population had higher population biomass and stronger density-dependence in the presence of refuges. When refuges were present, cohort competition increased and cycles with short periodicity were seen. Large individuals were not only cannibals, but could successfully prey on other species. Small and large guppies were allowed to invade resident populations of Heterandria formosa. Small invaders failed while large invaders succeeded as predation from large invaders broke up the competitive bottleneck that the resident population imposed on juveniles of the invader.