An analysis of the noise associated with electronic charge injection in charge-coupled devices (CCDs) is presented, including the effects of diffusion currents. The capacitance of the injection circuit determines the charge-injection noise, and specification of the noise requires consideration of the quoted capacitance; for large charge packets, the noise has a characteristic variance of kTC/2 or kTC, depending on whether the capacitance is defined by microscopic (internal) or macroscopic (external) potentials. The variance is Poissonian for small charge packets. The noise values presented in this paper may serve as a target when optimizing charge-injection operation of a CCD