In 1999, three groups independently developed a novel class of organic-inorganic nanocomposites known as periodic mesoporous organosilicas (PMOs). The organic functional groups in the frameworks of these solids allow tuning of the surface properties and modification of the bulk properties of the material. This paper provides a comprehensive overview of PMOs and discusses their different functionalities, morphology and applications, such as catalysis, drug delivery, sensing, optics, electronic devices, environmental applications (gas sensing and gas adsorption), biomolecule adsorption and chromatography for which PMOs have been used over the past 5 years.