Humans and other animals differ in suites of correlated behaviours commonly referred to as “personality traits”. While differential psychologists have a long tradition of studying human personality and its implications, behavioural biologists and evolutionary ecologists have more recently started to focus upon why animals possess individual personalities. This confluence of interests in personality across disciplines is providing novel insights. In this talk, we explore the areas in which psychologists and behavioural biologists now share common interests. We primarily focus on recent theoretical and empirical advances in the field of personality, outlining (i) why the existence of “personality” is generally non-intuitive from an evolutionary perspective, (ii) under which conditions inter- and intra-situation consistency might nevertheless be adaptive, and we finish with (iii) empirical evidence for adaptive population differences in personality constructs. This last section involves insights from recent field and laboratory work we have carried out on three-spined stickleback. In doing so, we provide strong evidence in favour of the view that personality variation is not ubiquitous, but instead is likely to evolve only in particular types of environment (i.e. those containing piscivorous predators). It is also clear that there is a genetic basis for personalities as well as for the phenotypic plasticity in personality that we demonstrate as a result of early-life experience for stickleback fry. Such apparently adaptive variation in personality provides an excellent starting point, along with a methodological framework, for understanding when and why human and animal personality might exist.