The effects of pharmaceutical excipients on the quality of medicinal products are discussed. It is emphasized that excipients must be studied before the development of a product starts, including compliance with production standards, investigations of their influences on the physical, pharmacological, and microbiological characteristics of products, and influences on the therapeutic efficacy and safety of medicinal formulations (release of active ingredient, bioavailability, etc.). Aspects considered in the present review indicate the need for a wide approach to assessment of the roles of excipients in determining the required pharmaceutical and therapeutic properties of medicinal products.