A battlefield; an abusive parent; the ongoing struggle to make ends meet; a pile-up of unanswered e-mails — stress can take many forms. But stressful situations have something in common: they trigger reactions such as fear and a surge of hormones. These responses can be beneficial in the short term but, if severe or prolonged, can damage the mind, triggering conditions ranging from depression to post-traumatic stress disorder. That much has been known for decades; now researchers are getting to grips with how stress can alter the biology of the brain, and tip a mind into illness. Here, Nature takes a look at what they have learned, and at the gaps that remain.