An advantage of long acting implants and injections is their ability to maintain a relatively constant drug concentration at the site of interest. In vitro drug release testing, particularly where an in vivo/in vitro relationship has been established, can help in ensuring in vivo performance. There are various in vitro release testing methods, which are chosen, based on availability, dosage form specifications and drug properties. These are categorized into three groups: sample and separate, flow-through and dialysis. Although there are currently no standard methods for in vitro release testing of controlled release parenterals, standard dissolution apparatus specified by the United States Pharmacopeia have been adapted in some cases and the development of compendial monographs is anticipated. An in vivo/in vitro correlation can predict the bioavailability characteristics of a product based on release in vitro.