This book represents deep ideas about equipment used mainly to detect the radioactivity and presence of the radioactive isotopes in the different products consumed by human. This equipment is called as Gas Proportional Counter. The central laboratories which are responsible of analysis of these products contain this equipment. The analysts who use this technique cannot solve some problems due to complications in some details about the instrument. So, we tried to show some of these details to provide the undergraduate students in the scientific colleges with some Knowledge about detection of the radioactive elements and provide the analysts and the researcher student who use this technique in their researches with some detailed information about this equipment to overcome the problems which face them during the measurement. Most of the material in this book will be suitable for teaching the undergraduate classes in the nuclear chemistry.