Michael Hassell’s work focuses on the dynamics of interacting populations, particularly insect host–parasitoid systems. He has pioneered an approach which combines carefully designed laboratory experiments with theoretical models and synoptic accounts of other field and laboratory data to elucidate how individual biological components of the interactions between and within species affect the persistence and abundance of populations. Importantly, he has drawn together theoretical models, short-term experiments and long-term population studies to show that the interplay between spatial heterogeneity and behavioural variability within populations has a crucial influence on the dynamics of many insect populations.