The present volume begins, in a summary fashion, with a description of the natural occurrence of the element Th. Especially stressed are those facts that are most important in understanding its geological distribution on Earth and its behaviour in minerals. Attached is a tabulation of the highest reported Th or ThO2 content in minerals that normally (by their crystal-chemical formula) do not contain Th. The main part of the present volume describes the minerals of Th. As can be seen from the crystal-chemical formulas, there are, in addition to a few minerals containing Th as the sole cation, a number of minerals that contain Th as an additional cation or as a diadochic element. In the case of diadochic substitution, the mineral may present a Th-rich end member of a solid-solution series and, therfore, is described as a separate Th mineral, or may only sporadically contain higher amounts of Th (no mineral description is given). The mineral descriptions in this volume - including oxides, carbonates and phosphates/siliconphosphates of Th - comprise the following topics: occurrence; chemistry; crystal form and structure; optical and other physical properties; and chemical and thermal behaviour.