This document discusses sustained and controlled drug delivery systems. Sustained release drug delivery systems aim to prolong the therapeutic effect of a drug through continuous release over a period of time from a single dose. Controlled release systems automatically deliver drugs at predetermined rates over long periods to maintain constant drug levels. The key difference is that controlled release systems achieve zero-order kinetics to precisely control drug levels, while sustained systems do not necessarily achieve this. Controlled release provides benefits like reduced dosing frequency, fewer side effects, and improved patient compliance. Challenges include potential for dose dumping and stability issues. The rationale is that these systems can deliver drugs locally or target delivery through spatial and temporal control of release.