In our modern life the gamma rays has a widespread applications in all fields of life and introduce to our modern civilized society generous facilities, some are supplementary while others are essential and can't be substituted easily. In contrast, the risk is always accompanying dealing with gamma rays (or any ionizing radiations) and care and caution is required. This book is concerned with the accurate calibration of the simple shape of gamma camera with sources of different shapes (regular and irregular). A set of closed mathematical expression is derived to calculate the efficiency of the gamma camera based on the efficiency transfer technique (ET) - that require a prior simple calibration procedure using a point standard source - using sources having different regular shapes (cylindrical, parallelepiped and spherical) and also using an irregular thyroid-like shape composed of two spheres in contact. The effect of the source self absorption was taking into account as well asany attenuator between the source and the active volume of the gamma camera.