In neuroscience, the brain reward system is a neural circuit that induces the feeling of pleasure when triggered by something we enjoy, like food or good music. The reward pathway has evolved to fulfill man’s need for survival such as eating and mating. Healthy ageing is usually accompanied by cognitive impairments and dysfunction of the dopaminergic system. Similar observations are made in several neurological and psychiatric disorders. At the same time, stroke-induced damage and neurodegeneration in dementia may be retarded by oestrogen. Although the effect of oestrogens on preventing ageing-related loss of cognition is well established, little is known about the effects of ageing and hormone replacement therapy (HRT) on the reward system. Addressing this issue was the key objective of the EU-funded project AGING Hormone. More specifically, scientists wished to study the effects of gonadal steroid hormones on reward processing and decision making in older women and healthy young men. Combining functional magnetic resonance imaging (fMRI) and endocrinology, early post-menopausal women were studied after placebo and after HRT (17-beta estradiol and progesterone). Young men who had received placebo or testosterone therapy were also included in the study. Results in early post-menopausal women clearly showed a modulation of the reward system and cognitive control by hormonal treatment. In men, results indicated that testosterone had a differential effect on reward processing. Subjects administered with the hormone tended to punish unfair offers and to reward generous ones, suggesting that the effects of testosterone depended on the social situation. Collectively, AGING Hormone project results provided evidence of a neurofunctional modulation of the reward system and of decision making mechanisms by gonadal steroid hormones in humans. These findings have important consequences for our understanding of drug abuse vulnerability and sex-related neuropsychiatric and hormonally-mediated mood disorders.