This book covers the main physical mechanisms and the different contributions (1/f noise, shot noise, etc.) behind electronic fluctuations in various spintronic devices. It presents the first comprehensive summary of fundamental noise mechanisms in both electronic and spintronic devices and is therefore unique in that aspect. The pedagogic introduction to noise is complemented by a detailed description of how one could set up a noise measurement experiment in the lab. A further extensive description of the recent progress in understanding and controlling noise in spintronics, including the boom in 2D devices, molecular spintronics, and field sensing, is accompanied by both numerous bibliography references and tens of case studies on the fundamental aspects of noise and on some important qualitative steps to understand noise in spintronics. Moreover, a detailed discussion of unsolved problems and outlook make it an essential textbook for scientists and students desiring to exploit the information hidden in noise in both spintronics and conventional electronics.