This book is intended for undergraduates who take a third and fourth quarter of quantum physics, and thus limits itself to those topics that are absolutely necessary for understanding elementary particle physics and condensed matter. The book consists of two parts: the first part presents radiation theory, scattering, symmetries, relativistic quantum physics, and special topics; the second part presents second quantization, covariant perturbation theory, and simple applications to the weak interaction and quantum electrodynamics.