Quantum-chemical theories of atomic and molecular structure are now part of the upper secondary curriculum in many countries, despite the fact that many educators are against their use in basic chemistry courses. Quantum-chemical theories of atomic and molecular structure are taught in general chemistry or introductory inorganic chemistry courses in chemistry and other science departments. Besides, atomic and molecular orbitals are part of the upper secondary curriculum in many countries. The first step in solving a quantum chemical problem is usually solving the Schrödinger equation (or Dirac equation in relativistic quantum chemistry) with the electronic molecular Hamiltonian. This is called determining the electronic structure of the molecule. It can be said that the electronic structure of a molecule or crystal implies essentially its chemical properties. This book provides extensive account of recent developments in the field. Attempts have been made to design a textbook by incorporating important topics that are useful and effective for students.