In the 1930s scientists discovered that the Universe is expanding and that it is quite old. The observation of hydrogen lines in the spectrum of the Sun helped Niels Bohr construct his atomic model in 1912, and understand the optical spectra of atoms. This era marked the transition of astronomy into astrophysics. With the rapid technological progress, scientists were able to study the universe in different ways that enabled them to observe what could not be observed using ordinary telescopes. Technology enabled scientists to see the universe in x-rays, gamma rays, radio waves and even look inside stars with neutrinos. One of the most important results of these developments, the observation of very high energy particles from cosmic distances, led to a complete new branch of physics, namely high energy physics, and provided a valuable tool to understand the very high energy processes going on in the universe such as in shock fronts of supernova remnants. In the 1960s very important discoveries in astrophysics like the microwave background radiation from the Big Bang, quasars, X-ray binaries, pulsars and cosmic X-ray sources followed. A significant interest in astrophysics prevailed and many physicists began to work on these new objects and processes. Among many great physicists, Yakov B. Zeldovich and Igor D. Novikov are worth noting especially because of their efforts which accelerated the theoretical and phenomenological researches in all of these branches of astrophysics. Astrophysics began to push the boundaries of physics and our world view. Astrophysical results were rewarded with Nobel prizes. Three of these Nobel prizes (1974, 1993, 2002) were given to works on neutron stars. This book is mainly devoted to neutron stars and to objects related to them.