Excipients are components of a finished drug product other than the active pharmaceutical ingredient (API) and are added during formulation for a specific purpose.  In contrast to active ingredients, minor components of an excipient may have significant impact on its pharmaceutical performance. They are included in dosage forms to aid manufacture, administration or absorption. Excipients as like other active pharmaceutical ingredients need to be stabilized and standardized. They act as protective agents, bulking agents and can also be used to improve bioavailability of drugs in some instances. Specific excipients are best suited for a particular dosage form; the selection criterion for excipients and various interactions that an excipient can undergo during its course of stay in formulation has been discussed in this review.