The purpose of this book is to present a state of art summary of current knowledge of methods of assessment of radionuclides in the terrestrial and marine environments. It cover the traditional methods of radioactivity measurements such as radiometrics techniques, but also recent developments in the mass spectrometry sector. The book starts with a short preface introducing the subject of the book, summarising content and philosophy of the book, as well as the most important historical achievements. The scientific topics are introduced by description of sampling methods, optimisation of sampling sites and sampling frequency. The recent developments in radiochemical separation methods using chromatography resins for the treatment of actinides, transuranics and other groups of radioelements are also described. No other book is available covering all aspects of environmental radioactivity measurements, although remarkable progress has been made in detection techniques over the last ten years. At present the new methods enable to carry out investigations which were not possible before, either because of lack of sensitivity or because of the fact that they required too large samples.