Why are fishes more abundant in some areas than others? Why do abundances vary over time? What factors limit the sizes and productivity of populations? How do natural populations respond to harvesting, and where do we draw the line between sustainable and unsustainable fisheries? These are some of the questions that motivate researchers, and they underpin important issues in conservation and management. The answers might easily command a book, but our focus, of course, is freshwater fishes and fisheries in Australia. The size of a population is determined by interplay between the environment and four processes, namely recruitment, mortality, immigration and emigration. To understand how these interact, we need information about movements (Chapter 5), trophic dynamics (Chapter 6), life histories and recruitment (Chapter 7), age and growth (Chapter 8) and the influence of alien species (Chapter 11). Here, we draw on these topics to illustrate the dynamic changes that occur in patterns of abundance, recruitment, mortality, migration, growth and production. We introduce some concepts, then consider population models and the impacts of fishing.