The book focuses on the principles of electronic and magnetic properties of transition and inner transition metal ions/complexes. It is one of the essential and fundamental topics for graduate and undergraduate students, and it is also important for researchers. The book also covers basic concepts of symmetry in orbital, point group, spectroscopic terms and energy level diagrams. Crystal field, ligand field and molecular orbital theory (along with their importance in spectra) are elaborated. Further magnetic properties of transition metal ions/complexes as well as their spin and orbital contributions have been described in this book.