This book deals with the phenomena of magnetism exhibited by intermetallic compounds. We selected the rare earth based intermetallic compounds considering the interesting magnetic behavior of the 4f orbital. A variety of interesting properties are exhibited in these compounds simply by changing the rare earth element. The interesting behavior includes: Pauli-paramagnetism, heavy fermion behavior, crystal field effects, hard and soft ferromagnets, various anisotropic effects, etc. Apart from this, exploring the 4f orbital has become a trend for the fundamental studies as well as practical applications. The presence of a 3d orbital also motivates the interest because of the interplay between them. We present our studies on 4f orbital, 3d orbital and their interplay in this book.