This book examines our current understanding of the population dynamics of one kind of interaction - that between insect parasitoids and their hosts. Parasitoids are amongst the most abundant of all animals, and make up about 10% or more of metazoan species. Almost no insect species escape their attack. Host-parasitoid interactions were first modelled over fifty years ago, but for many years there was little good empirical information on the important factors that affect host and parasitoid populations. The models were very simple, and their predictions rather divorced from the complexity of what was visible in the field. Now, better data is available on many components of host-parasitoid systems, from field observations and laboratory and field experiments, and this allows a much closer correspondence between models and data. In particular, the past twenty years have seen major advances in our understanding of how host-parasitoid interactions are influenced by spatial processes, by age-structure effects, and by competition from additional host and parasitoid species. The result is a body of theory that makes direct contact with real systems in the field, and provides us with a detailed understanding of what underpins a whole area of population dynamics. In this book, Michael P Hassell pulls the theory and field data together to present an elegant illustration of the way in which ecological studies advance.