For many applications, polymer microspheres (MS) should possess a monodisperse size distribution. With such uniformity they are able to deliver precise amounts of drug per MS, optimize the release kinetics of an encapsulated drug, obtain repeatable in vivo biodistributions to different organs and tissues, and obtain the maximum protection of (protein) drugs from degradation. This review classifies monodisperse polymer MS according to their methods of production and gives examples of the formation of uniform MS and their applications in the medical field. In the literature, the term ‘monodisperse’ is often used inaccurately, and this article attempts to rectify this by clearly defining monodispersity in terms of the coefficient of variation and the polydispersity index, the two statistical quantities most frequently used to describe the size distribution of MS.