For specific technological applications, it is very important to control the electrical properties which can be adjusted by doping process. In general, the doping process is achieved by adding the dopants directly to the growth ampoules . It is quite hard to control the amount of impurity due to segregation and solubility problems. The ion implantation technique is simply the launching of atoms onto the surface of the material by bombardment of the solid with energetic ions and does not render these difficulties. The main advantage of this technique is that the doping atoms can be implanted in different concentrations, independent of their solubility in the target materials. Therefore, the material properties can be controlled by using the implantation technique. In this work, the effect of N- and Si-implantation on structural, electrical and optical properties of GaSe single crystals were extensively studied. This book is produced from my Ph.D thesis completed in the Middle East Technical University, Ankara, Turkey in 2003.