Book excerpt: Gives basic and up-to-date information about noise sources in electronic devices. Demonstrates how this information can be used to calculate the noise performance, in particular the noise figure, of electronic circuits using these devices. Optimization procedures, both for the circuits and for the devices, are then devised based on these data. Gives an elementary treatment of thermal noise, diffusion noise, and velocity-fluctuation noise, including quantum effects in thermal noise and maser noise.