Closed fullerenes encapsulating small molecules have been synthesized. Representative are the synthesis of the dihydrogen endofullerene H@C, the water endofullerene HO@C, the hydrogen fluoride endofullerene HF@C, and the methane endofullerene CH@C. The encapsulated molecules display unusual physical properties which have been studied by a variety of physical methods. As shown theoretically, compression of molecular endofullerenes (e.g., H@C) may lead to dissociation of the encapsulated molecules and reaction of their fragments with interiors of the fullerene cage. Such reactions should result in endohedral fullerene adducts, which are currently unknown.