This chapter presents an overview on the different types of drug delivery systems that are predominantly governed by diffusion. The systems are classified according to their physical structure (reservoir devices versus monolithic systems), as well as according to the ratio of “initial drug loading and drug solubility.” For various cases, mathematical models are briefly presented considering different device geometries. These theories are mechanistically realistic and enable quantitative description of the resulting release kinetics. Effects of formulation parameters on drug release can also be predicted quantitatively. Practical examples are given to illustrate the applicability of the presented theories and the benefits of understanding how a diffusion-controlled drug delivery system works.