Mindful of possible future limitations on the availability of uranium, the introduction of the thorium fuel cycle is potentially a complementary source of nuclear energy. This publication assimilates current knowledge of thorium geology and mineralization into a brief account on the worldwide occurrence of thorium resources. Although thorium is currently not commercially viable as a fuel, it is important to pre-emptively assess thorium related information should that situation change. Thus, the publication provides an overview of the variety of natural thorium deposit types with associated thorium geology and thorium resources. It reviews available data on thorium occurrences/deposits and thorium resources and presents a classification of deposits according to geological and economic criteria.