This chapter provides recommendations for the in vivo and in vitro assessment of solid oral drug product performance. The chapter is intended as a guide for scientists and clinicians seeking to evaluate drug product performance by surrogate procedures correlative and/or antecedent to clinical trials in humans. USP–NF provides quality standards for drug substances, excipients, and finished preparations. A USP–NF monograph for an official substance or preparation includes the article's definition; packaging, storage, and other requirements; and a specification. The specification consists of a series of universal tests (description, identification, impurities, and assay) and specific tests—one or more analytical procedures for each test—and acceptance criteria. Quality standards are important attributes that must be built into the drug product. Meeting USP–NF standards is accepted globally as assurance of high quality and is part of the requirements necessary for approval of a bioequivalent , interchangeable drug product. Drug products must meet certain in vivo and/or in vitro performance standards to be considered therapeutically equivalent (TE) and interchangeable. Drug product performance may be defined as the release of the drug substance from the drug product dosage form, normally leading to systemic exposure or less often to local activity of the drug substance necessary for achieving a desired therapeutic response. Bioavailability (BA) is a measure of systemic exposure. This chapter discusses in vivo and in vitro approaches to determining drug product performance. The focus of the chapter is primarily on the performance of solid oral drug products.