Diatomic molecules have an importance because of their existence in our world. So, this book attempts a comprehensive presentation of the field of spectra of diatomic molecules. This theoretical work concerns with the spectroscopic parameters of diatomic molecules such as calculations of Einstein's coefficients A , Franck- Condon factors , the rotation effect upon Morse potentials, the rovibronic spectra of these molecules, transition probabilities and Boltzmann distribution(statistical treatments) for PQR branches. A discussion of the these parameters of some diatomic molecules had been done and followed by a detailed treatment of electronic transitions. The book provides an insight on the basic physical properties of some diatomic molecules.