This document summarizes ophthalmic drug delivery systems. It discusses how absorption of eye drugs depends on physicochemical properties and tear drainage. Frequent dosing of short half-life drugs can cause irritation and side effects. One approach to improve effectiveness is prolonging drug contact with the cornea. The best known system is the ocular insert or ocusert, which provides controlled drug delivery over 7 days through rate-limiting membranes, in contrast to eye drops dosed 3-4 times daily.