Populations of insects can fluctuate dramatically through time in response to changing environmental conditions. Disturbances are particularly important to population dynamics, triggering outbreaks of some species and locally exterminating others. Disturbances affect insect populations directly by killing intolerant individuals or indirectly by affecting abundance and suitability of resources or abundance and activity of predators and parasites. Population growth can be regulated (stabilized) to a large extent by density-dependent factors whose probability of effect on individuals increases as density increases and declines as density decreases. Primary density-dependent factors are intra- and interspecific competition and predation. Increasing competition for food (and other) resources as density increases leads to reduced natality and increased mortality and dispersal, eventually reducing density. Similarly, predation increases as prey density increases. Populations declining below their extinction threshold may be doomed to local extinction, whereas populations increasing above a release threshold continue to increase during an outbreak period. Development of population dynamics models has been useful for forecasting changes in insect abundance and effects on crop, range, and forest resources. General models include the logistic equation that describes a sigmoid curve that reaches an asymptote at carrying capacity. Chaos models have addressed the importance of initial conditions for subsequent changes in population size. More complex models incorporate specific population variables, including key factors and time lags. Despite limitations, models represent powerful tools for synthesizing information, identifying priorities for future research, and simulating population responses to future environmental conditions.