Purpose  Evaluate various ocular drug delivery systems and emphasize the advantages and disadvantages toward specific purpose. 
Methods  From anatomical point of view, ocular drug delivery systems are classified as episcleral delivery system, intra scleral delivery system, super choroidal space delivery system, intra choroidal delivery system, subretinal delivery system, and intravitreal drug delivery system. From formulation point of view, ocular drug delivery systems can be classified as implant, colloidal system, and vectorial system.
Results  Intravitreal drug delivery systems include intravitreal implant and intravitreal injection. This route of ocular drug delivery is a main stay at the current. However, surgical procedures and high cost associated with implants have limited its application. Intravitreal injection is less invasive but frequent injections amplify the risk of ocular infection and lower the quality of life. Subretinal drug delivery is invasive and therapeutic benefit for the near target comes on the cost of sacrifice of retina which directly hosts the delivery system. Intra choroidal drug delivery is an emerging method and lack of evidence for a scientific evaluation. Super choroidal space drug delivery including cannulation or injection has gained momentum. It avoids entering into eye cavity but it is not necessarily less invasive. So far many studies show its drug delivery efficiency is low. Transscleral drug delivery is a much less invasive route and many molecules including proteins can be delivered into retina this way. The key is to understand the drug transscleral transport mechanism to design optimized release kinetics of transscleral drug delivery device and drug formulation. A significant progress has been made in this area recently. From drug formulation view point, implant is dominant the market but colloidal drug delivery and vectorial drug delivery have made significant progress from research field to bedside. Most recently a very novel porous silicon particle vectorial drug delivery demonstrated high promise. It can be used for both intravitreal injection and episcleral plaque drug delivery. Daunorubicin has a few hours vitreous half-life, however, after covalently loaded into porous silicon particles its vitreous half-life can be extended to weeks. Avastin has been successfully loaded into these particles and one intravitreal injection can be expected to provide an at least 4 months therapeutic effect. More importantly, this delivery system provide a noninvasive remote release sensing ability through which the particle color or a wave length reader can readily tell how much drug is still in the system. 
Conclusion  Local drug delivery systems should be fully understood by eye care physician and porous silicon ocular drug delivery is a state of arts to deliver and monitor drug release in living eyes with minimum invasiveness. The clear media of human eye provide an excellent arenafor ocular drug delivery and sensing.