This Symposium held under sponsorship of Southern Research Institute in Birmingham, Alabama, April 19 and 20, 1973. The announced purpose of the symposium was to encourage an exchange of information among the experts working in various fields of controlled release and the scientists and technologists interested in applying the concepts. The number of registrants (over 120), the diverse nature of the organizations represented, and the enthusiastic participation of attendees in the discussions testified to intense and broad interests in controlled release. The papers presented at the symposium should serve well to introduce the principles of controlled release and demonstrate a few of the promising applications. Controlled release is an important step toward improving the delivery of a biologically active agent to its target. Precise administration of an agent can substantially reduce the concentration required for beneficial effects and thus minimize deleterious effects to the organism or to the environment. Through controlled release, older agents whose efficacies are established may prove more reliable, and newer agents whose high potencies or low stabilities have inhibited use may prove more suitable. Controlled release therefore offers both an alternative and a complementary route to the increasingly costly and demanding search for agents of greater specificity. The papers in this book appear in the order of their presentation at he symposium. The papers may not be identical to the ones presented at the meeting, however, because some of the papers had been condensed by speakers to fit the time allotted, and some of the manuscripts were changed by authors to clarify statements or answer questions raised in the meeting. Moreover, we chose to alter some of the symbols and equations to improve clarity, especially where these had been used to express diverse meanings.