In connection with the recent treatment of radium and the actinides, the Gmelin Institute is carrying out the description of thorium and its compounds. The supplement volumes A 2, A 3 and A 4 with the history, isotopes, uses, the recovery of thorium and general properties of thorium atom and ions, the thermodynamics of its compounds and solutions, and spectroscopic data have already been published. The supplement vol- umes C 1, C 2 and C 3 describing the compounds with the noble gases, hydrogen, oxygen compounds and nitrogen compounds are also available; also has been pu- blished Supplement Volume C 5 describing the compounds with sulfur, selenium, tellurium, and boron. The Supplement Volume D 1 and D 2 describing the properties of thorium ions in solution and the solvent extraction of thorium as well as Supplement Volume E de- scribing the coordination compounds also have been published. The present supplement volume A 5 of the Gmelin Handbook "Thorium" is devoted to the analytical chem- istry of this element, to its biological behavior and to health protection and safety control, including the monitoring of occupational exposure received by personnel. The analytical chemistry of thorium relies mainly on the so-called "classical" determination methods like gravimetric, volumetric, and spectrometric methods. Radiometric methods find also large application in the analytical chemistry of thorium. Presently we have a good understanding of the biological behavior of this danger- ous radioelement, together with a broad knowledge of its metabolism and its effects on humans. The therapeutic decorporation of thorium is also treated in this volume.