Insomnia is a highly prevalent sleep disorder, known to affect psychological well-being and quality of life. While perpetuating factors have received much attention, the role of predisposing factors has not been studied in much detail. The susceptibility to develop insomnia may be linked to the presence of certain personality features. Here, we review studies that assessed this particular aspect of insomnia. Due to various methodological issues, definitive conclusions cannot be drawn as of yet, and several conflicting findings remain. However, there is a common trend indicating that insomniacs display more signs of 'neuroticism', 'internalization', anxious concerns and traits associated with perfectionism. These factors may play varying roles depending on the specific subdiagnosis of insomnia. In addition, certain personality traits may be related to the response to (cognitive) behavioral treatment. For instance, insomniacs reporting less 'guardedness' and have a higher score on the MMPI 'hypomania' scale show less improvement through psychological treatment. The specific role of personality traits in the etiology of insomnia is not yet clear, because of a lack of longitudinal data. Personality factors may play a causal role in the development of insomnia, but may also be a consequence of the sleep problem and the associated daytime dysfunction. Future longitudinal studies should not view personality as a single predisposing factor, but assess it as a part of a larger group of interacting psychological and physiological factors involved in the predisposition to and perpetuation of chronic insomnia. Copyright 2009 Elsevier Ltd. All rights reserved.