5-Hydroxytryptamine (5-HT, serotonin) has long been implicated in a wide variety of emotional, cognitive and behavioural control processes. However, its precise contribution is still not well understood. Depletion of 5-HT enhances behavioural and brain responsiveness to punishment or other aversive signals, while disinhibiting previously rewarded but now punished behaviours. Findings suggest that 5-HT modulates the impact of punishment-related signals on learning and emotion (aversion), but also promotes response inhibition. Exaggerated aversive processing and deficient response inhibition could underlie distinct symptoms of a range of affective disorders, namely stress- or threat-vulnerability and compulsive behaviour, respectively. We review evidence from studies with human volunteers and experimental animals that begins to elucidate the neurobiological systems underlying these different effects.