We had previously observed, when cosputtering from a single target, that the deposited film was far from uniform, even showing a pattern resembling that of the target. Having subsequently made some energy and power measurements of the negative particles bombarding the anode in a sputtering system, we have concluded that the patterning is due to fast secondary electrons from the target. These secondary electrons are of importance in all types of sputter deposition, since a significant number of them travel in straight lines to the anode without collision. They are responsible for almost all of the power input into the anode and can dominate the growth of a thin film on a substrate at the anode.