A brief introduction to spintronics, including the classification of the research fields. - The main focus is the 2nd category to which semiconductor-based and logic devices belong. • Two important technical bases for spintronics, the two spin-channel model and DOS/mobility asymmetry, and some basic equations including the drift-diffusion equations for the current flow. • The applications of these fundamentals to spin injection; Weak or no spin injection unfortunately. • Possible solutions to efficient spin injection: tunnel barrier, half-metals, and DMSs. • Two main techniques of spin detection: electrical and optical detection. - Electrical detection in the nonlocal geometry and the optical detection in the spin-LED structure are particularly good. • Some recent advances on MgO-based MTJs and STT. • STT-MRAM and Racetrack Memory are of particular practical importance.