One of Tinbergen's most lasting contributions to the study of behaviour was the distinction he drew between causal, functional, developmental, and evolutionary questions about behaviour. More recently, behavioural ecologists have claimed that understanding the function of behaviour is an important step towards understanding its causes. This claim has, in turn, been criticised for confusing the fundamental distinction that Tinbergen defined. The study of behaviour, however, usually begins by identifying units of behaviour functionally and only then proceeds to causal analysis. Research carried out on four phenomena — disassortative mating by MHC loci, memory for cache sites in food-storing birds, auditory localisation of prey by barn owls, and magnetic orientation — illustrates the contributions made to causal research through understanding the function of behaviour. Understanding function, and sometimes simply a hypothesis about function, defines the causal questions that are asked, identifies novel questions for causal investigation, and sets the criteria that causal explanations must satisfy.