The adhesion of toner particles charged by triboelectricity plays an important role in the electrophotographic process. In spite of the importance of this phenomenon to electrophotography, the physics of toner adhesion is not well understood. A literature survey of toner adhesion measurements reveals adhesion forces which are typically 5 to 50 times larger than the predictions of the electrostatic image force model, a description which treats the irregularly-shaped toner particles as spheres with uniform charge density. To account for this discrepancy and other observed behavior, a toner adhesion model is presented which assumes a highly non-uniform surface charge distribution. This charge patch model explains the results of electric field detachment studies, which show enhanced adhesion and the absence of strong selectivity in toner charge and size.