This textbook has been specifically developed on the directives of National Education Policy (NEP 2020) and is as per latest curriculum of B.Sc. Physics (Sixth Semester) for the Universities of Uttar Pradesh. This book comprises of two parts: Part A deals with Introduction to Solid State Physics and it begins with Crystal Structure followed by Crystal Diffraction, Crystal Bindings and Lattice Vibrations. Part B on Introduction to Nuclear Physics starts with Nuclear Forces and Radioactive Decays followed by Nuclear Models and Nuclear Reactions, Accelerators & Detectors and Elementary Particles. All topics are lucidly explained in this book.