Individual differences in response to stressful experiences are a hallmark of the human condition. The same experiences that some people find aversive are considered neutral or rewarding by others. Paradoxically, experiences that are rewarding can also be defined as stressful because they activate stress hormone systems, such as the hypothalamic pituitary adrenal axis. Despite this activation, however, the brain is often buffered against the negative effects of high stress hormones when the experience is perceived as rewarding or "positive". The adult brain is structurally plastic, undergoing changes in the birth of new neurons, a process called neurogenesis, and remodelling of dendrites. Positive and negative stress can modulate brain structure and these changes are believed to participate in cognitive function (the processes of perception, memory, judgment, and reasoning) and mood regulation. In this lecture, Professor Gould will discuss the influence of stress hormones on plasticity in the adult brain under aversive and rewarding conditions. Negative stress inhibits adult neurogenesis and results in atrophy of some types of neurons as a result of elevated levels of stress hormones. While positive stress results in even greater increases in such levels, these experiences are associated with brain growth. Professor Gould will argue that the effects of stress on the brain are complex and can be mediated by the social context, which may buffer the brain from negative consequences.