Emulsion is a biphasic liquid dosage form. An emulsion is a dynamically ambiguous system subsisting of partially two non- miscible liquid state, upon which is diffuse as drop (the dispersed phase) in the other liquid state (the continuous phase), preserved by the existence of an emulsifying agent. The molecule diameter of the diffuse states usually develop from about 0.1 to 10 μm, despite particle diameters as slight as 0.01 μm and as extensive as 100 μm are not infrequent in some formations.These are of two type oil in water and water in oil , it is a conventional method now days novel dosage forms are used like microemulsion and nanoemulsion. These are prepared by using different process and provide better bioavilability and response .There size are also differ and provide good result as comparison of emulsion.Novel drug delivery system is commonly used having better approach towards other dosage form. Emulsion is one of the most important system  of dosage form . Applications of emulsions increased especially after micro and nano-emulsion emergence. This paper is an attempt to summarise comparative aspects like definition, theories, types, methods of prepara­tions, advantages, disadvantages and methods of analysis of emulsion, micro-emulsion and nano-emulsion. Microemulsions are also known as transparent emulsion, swollen micelle and micellar solution. self-microemulsifying drug delivery system (SMEDDS) is also one of the popular term for microemulsion mediated delivery of drugs.

Keywords-Emulsion, Microemulsion, nanoemulsion, dry gum method,shear force