Developments in recent drug discovery programs, yields a large proportion of novel pharmacologically active molecules that are lipophilic and poorly soluble, which is a major challenge for pharmaceutical researchers to enhance the oral bioavailability of such drug molecules. The self-nanoemulsifying drug delivery system (SNEDDS) is a promising Drug Delivery System which is well known for its prospective to improve the aqueous solubility and oral absorption of poorly water soluble drugs. SNEDDS is an isotropic mixture comprising oil, surfactant, co-surfactant and drug that form oil in water emulsion in aqueous environment under placid agitation. It can readily disperse in the aqueous environment of the gastrointestinal tract to form a fine oil-in-water emulsion with a droplet size not exceeding 100 nm under mild agitation for improving the oral bioavailability of poorly water-soluble drugs. The self nano-emulsifying Drug Delivery System (SNEDDS) is applicable on BCS Class II and Class IV Drugs for improving water Solubility of poorly water soluble drugs. It is the novel drug delivery system which is applicable for oral, parenteral, ophthalmic, intranasal and cosmetic delivery of drugs. The review presents an overview of design of formulation, preparation of components, mechanism of selfemulsification, biopharmaceutical aspects and evaluation of self nano-emulsifying drug delivery system (SNEDDS) for enhancement of oral bioavailability of poorly water soluble drugs.