An increasing body of information tends to support the hypothesis that drug effect, therapeutic or toxic, is more closely correlated with plasma concentration than dose (Levy, 1968). Absorption is one important determinant of drug plasma levels. Distribution and elimination are others. Most drugs are administered as drug products, not drug entities. The biologic performance of a drug product can be affected by its bioavailability (defined as a measure of the rate and extent to which a drug reaches a sampling site (usually a peripheral vein) or its site of action). Biophasic availability is generally reserved for situations where pharmacologic data are used to assess biologic performance (Smolen, 1971). This paper discusses the various factors that can influence the bioavailability of drug products. Major emphasis is placed on preparations intended for oral administration.