This paper reviews the literature pertaining to seasonal patterns in psychiatric illnesses. Evidence on the season of birth phenomenon suggests that a greater risk of pre-, peri-, or post-natal damage is associated with the winter months. There is currently insufficient evidence to pinpoint the exact mechanism involved or to ascertain whether the mechanism is common to each condition. Studies of seasonal patterns of incidence of psychiatric disorders have highlighted the role of seasonally regulated environmental factors on internal biological processes. There is growing evidence that serotonin is involved in a variety of psychiatric disorders. Seasonal patterns have been observed in processes involving serotonergic functioning. Furthermore, it has been postulated that these processes may be influenced by photoperiod, suggesting that the seasonal patterns of incidence of several psychiatric conditions may share a common neurophysiological substrate.