Genetics and evolution have been said to be the two major unifying themes in biology. This lecture is intended to provide and introduction to evolutionary theory and how it can account for biological diversity and the evolution of behaviour in primates and other animals. After first thinking about the nature of DNA and mutation - the starting point for genetic variation - we will think about random genetic drift and natural selection. We will then turn to instinctive behaviours. When a spider spins a web or a honeybee constructs a honeycomb, such complex, instinctive behaviours seem hardwired into these animals, though the work of Erasmus Darwin and Jean-Baptiste Lamarck suggest that instinct may also arise from environment. Whilst evolution shapes such behaviour the fine details of the genetic mechanisms leading to behavioural variation are, as yet, not fully understood. Thus, more complex animal behaviours such as mate choice, parenting, social organisation, co-operation and conflict are studied experimentally and in natural environments in an attempt to elucidate gene-environment interactions. Yet other animal behaviour is likely influenced by an animal’s own particular experiences, leading to considerable behaviour variation within a give species. Thus, scientists also study what and how animals learn from their environment. In the final part of this lecture, we will consider modern transgenerational epigenetic mechanisms underpinning some behaviours and critically examine whether theories of evolutionary psychology can aid in explaining how complex animal behaviour evolves.