Over the past decade, a rapidly growing number of studies have shown that animals often exhibit personalities; e.g., where some individuals are consistently more aggressive, bold, active, exploratory or social than others. Here, I present theory, data and ideas on three ‘frontiers’ in the study of the ecological and evolutionary implications of this phenomenon. First, I discuss a theoretical/conceptual framework for explaining variation in the phenomenon. Why are individuals sometimes very consistent (stable) in their personality, but other times, less so? When do we expect early experiences to have large effects on later personality, versus when do we expect little or no lasting effect of early experiences? Second, I examine the interaction between individual behavioural types (BTs) and the social situation in determining individual behaviours and fitness outcomes (mating success). This section raises issues about group selection on personalities, keystone individuals, social skill and rapidly reversible mating systems. Finally, I present data and ideas on a key ecological implication of behavioural syndromes – the effect of behavioural type dependent dispersal on ecological invasions and spatial ecology, in general.