This chapter highlights some of the key factors in designing and developing parenteral controlled-release dosage forms. It addresses some of the reasons why one would select a parenteral controlled-release dosage form over other controlled-release dosage forms. The chapter outlines the advantages and disadvantages of parenteral controlled-release dosage forms. It provides examples of some of the key approaches that have been taken to afford safe and efficacious parenteral controlled-release dosage forms. These examples range from older, but still useful, technologies involving emulsions, drug solutions, and drug suspensions to new and more sophisticated technologies such as biocompatible, biodegradable, injectable microsphere formulations. One of the key factors in designing an injectable microsphere delivery system is to choose the appropriate polymeric excipient. Rheologically, an injectable suspension presents a formidable task to achieve. Although an injectable suspension can be formulated to have desirable properties, rheological changes may occur during filling, shipping, injecting, and storage.