Mild cognitive impairment (MCI) is a recently described syndrome that is currently thought of as a transition phase between healthy cognitive ageing and dementia. Although this notion seems to be reasonable, the general nature of the term MCI—including its many definitions—makes accurate accounting of the prevalence, prognosis, and potential benefit from treatment somewhat difficult. The differences in cognitive profile and clinical progression among individuals with MCI are generally recognised. However, recent evidence also suggests that the aetiological heterogeneity among individuals with MCI could be greater than previously reported. For example, cerebrovascular disease seems to be underestimated as a potential cause of MCI. In this review, I attempt to recognise workable definitions of MCI to discuss the prevalence, pathophysiology, prognosis, and possibilities for treatment of this disorder.