The content and the application of pharmaceutical dosage forms must meet several basic requirements to ensure and maintain efficiency, safety and quality. A large number of active substances have a limited ability to direct administration. Excipients are generally used to overcome the limitation of direct administration of these active substances. However, the function, behavior and composition of the excipients need to be well known in the design, development and production of pharmaceutical dosage forms. In this review, excipients used to assist in any pharmaceutical dosage form production processes of drugs, to preserve, promote or increase stability, bioavailability and patient compliance, to assist in product identification/separation, or to enhance overall safety and effectiveness of the drug delivery system during storage or use are explained. Moreover, the use of these excipients in drug delivery systems is identified. Excipient toxicity, which is an issue discussed in the light of current studies, also discussed in this review.