Over millions of years, biological systems have evolved wide varieties of nanoparticles. Naturally occurring nanoparticles show great diversity: they may be intracellular or extracellular, formed of organic or inorganic materials and have wide-ranging biological roles. Despite this diversity, nanoparticles found in nature possess several characteristics that make them attractive for biomedical purposes. This review presents an overview of the most common biological nanoparticles and outlines the potential applications of natural and modified biological nanoparticles.