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The Biopharmaceutics Classification System (BCS) for oral immediate release solid drug products has been very successful; its implementation in drug industry and regulatory approval has shown significant progress. This has been the case primarily because BCS was developed using sound scientific judgment. Following the success of BCS, we have considered the topical drug products for similar classification system based on sound scientific principles.In USA, most of the generic topical drug products have qualitatively (Q1) and quantitatively (Q2) same excipients as the reference listed drug (RLD). The applications of in vitro release (IVR) and in vitro characterization are considered for a range of dosage forms (suspensions, creams, ointments and gels) of differing strengths. We advance a Topical Drug Classification System (TCS) based on a consideration of Q1, Q2 as well as the arrangement of matter and microstructure of topical formulations (Q3). Four distinct classes are presented for the various scenarios that may arise and depending on whether biowaiver can be granted or not.