In a narrow sense spintronics refers to spin electronics, the phenomena of spin-polarized transport in metals and semiconductors. The goal of this applied spintronics is to find effective ways of controlling electronic properties, such as the current or accumulated charge, by spin or magnetic field, as well as of controlling spin or magnetic properties by electric currents or gate voltages. The ultimate goal is to make practical device schemes that would enhance functionalities of the current charge-based electronics. An example is a spin field-effect transistor, which would change its logic state from ON to OFF by flipping the orientation of a magnetic field. In a broad sense spintronics is a study of spin phenomena in solids, in particular metals and semiconductors and semiconductor hetero-structures. Such studies characterize electrical, optical, and magnetic properties of solids due to the presence of equilibrium and non-equilibrium spin populations, as well as spin dynamics. These aspects of spintronics give us important insights about the nature of spin interactions-spin-orbit, hyperfine, or spin exchange couplings solids. This Book is lucid presentation of Spintronics.