The many characteristics of nanoparticles (NPs) depend on their size and size distribution. When novel functions of NPs are attempted, it is necessary to monodisperse or nearly monodisperse NPs. Many synthesis methods have been proposed from solution-based chemistry to satisfy this demand. Despite such special techniques to prepare NPs, it is often the case that an additional classification procedure is needed to a certain extent to obtain monodispersed NPs, similar to other general precipitation methods used in NP preparation. Additionally, NPs must be purified post-synthesis to remove by-products. Hence, size-selective separation as post-synthesis operation is expected to become increasingly important. In this review, some recent developments of size-selective separation methods for NPs based on using external fields, filtration, and the stability of the colloid system are highlighted.