Many drugs have limited effectiveness when taken orally, so they are often administered parenterally. This allows the drug to enter the bloodstream directly without going through the first-pass metabolism. However, as frequent injections can cause side effects, sustained-release formulations that provide long-term drug delivery are developed and employed. Implants based on biodegradable polymers, such as preformed and injectable in situ forming implants, have become prevalent for both local and systemic applications due to their effectiveness and better patient compliance. This chapter provides an overview of implantable systems, including their benefits and drawbacks, with several examples of the available clinical and commercial applications.