Understanding of the molybdenum diselenide formation is important for the development of low resistance electrical back contacts for high efficiency solar cells, since thickness and crystallographic orientation of molybdenum diselenide layer determine the adhesion and electrical properties of the absorber-Mo interface. This book focuses on the selenization process of Mo samples prepared by different methods and different coverages: sputtered glass/Mo, Mo-foil, glass/ITO/Mo, glass/Mo/In, glass/Mo/Cu, glass/Mo/Cu-In and glass/Mo/Cu-Zn-Se were used as substrates to study the molybdenum diselenide layer formation. Dependence of the selenization process of Mo on the temperature and time was studied by the two-temperature zone and the isothermal one-zone selenization methods. Thickness, structure, morphology and composition of molybdenum diselenide were determined. Samples were characterized by EDX, SEM, XRD, XPS, Raman and by the Hot Probe method.