This book represents principles and concepts in types of spectra in molecules especially in diatomic molecules and explains the molecular physics, especially, I think this work useful for a third and fourth year undergraduate course in college of science. It presents concepts to an understanding of structure for molecules, it gives an important topics for student's interest has been assigned as a supplement and together offer an excellent grounding for students interested in graduate work, whether in this area or some other. Indeed, most of the students taking this course have gone on to study other areas of physics.