Nanoparticles have been doped or functionalized with photochromic molecules for a variety of applications that take advantage of the photoinduced changes in molecular properties that accompany a photochromic transformation. Many photochromic nanoparticles are designed to be fluorescent and to undergo reversible changes in fluorescence intensity or color upon irradiation. Such systems rely on the photochromic transformation to generate a fluorescence quencher. Other photochromic nanoparticle systems harness photoinduced changes in other molecular properties such as polarity and molecular size. This review features photochromic nanoparticles from the recent literature and gives examples of both organic and inorganic nanoparticles that have been doped or covalently functionalized with organic photochromic molecules.