Electronic noise is the collection of unavoidable spurious currents and voltages that are detected at the output of a semiconductor device even under quiescent conditions. These result from unavoidable physical process inside the material of the device. Electronic noise corrupts useful output signals especially when the input is itself low amplitude or the device is operating at 100s of MHz–10s of GHz (RF|microwave frequencies). This chapter examines in detail the numerous types of electronic noise, what causes them and how their signal degrading effects can be curtailed. All the state-of-art semiconductor industry standard device models—VBIC, Angelov Chalmers, BSIM etc., include device noise mechanisms.