Most GDDs have been developed in a virtual publication vacuum, with little available data to substantiate manufacturers' claims for flow performance or biocompatibility. Clinical data are largely restricted to uncontrolled retrospective case series with variable follow up and differing definitions of surgical success. Evaluation is further complicated by the heterogeneity of inclusion criteria. This review article highlights various GDDs, which will be a great help to ophthalmologists as well as post graduate students in Ophthalmology.