It has been argued that imprecise probabilities are a natural and intuitive way of overcoming some of the issues with orthodox precise probabilities. Models of this type have a long pedigree, and interest in such models has been growing in recent years. This article introduces the theory of imprecise probabilities, discusses the motivations for their use and their possible advantages over the standard precise model. It then discusses some philosophical issues raised by this model. There is also a historical appendix which provides an overview of some important thinkers who appear sympathetic to imprecise probabilities.