The drug delivery through the skin to achieve a systemic as well as local effect of a drug is known as Transdermal drug delivery system. Its main advantage includes controlled drug release with minimum side effects, improved bioavailability and many more. The human skin is readily accessible surface for drug delivery. For penetration of drug through the skin the stratum corneum is the main barrier. So to avoid the stratum corneum and to increase the flux through the skin membrane different techniques of penetration enhancement are used.  This review mainly represents different factors affecting Transdermal bioavailability, technologies for developing Transdermal drug delivery system, different evaluation tests for Transdermal patches, different methods of preparation of TDDS, recent techniques for enhancing Transdermal drug delivery system.