Individual differences in behaviour have long been considered noise around an optimal mean. Recently, however, personality has gained much more attention due to its ecological and evolutionary significance. Consistent (over time and context) inter-individual differences in behaviour are now being studies in species as diverse as water-striders, tits and primates. The latter, however, have mostly been studied using questionnaires filled out by researchers and keepers. Among others, I however, advocate a more objective and comparable (to for example water-striders) approach and use observational as well as experimental studies to investigate personality.