When the patent of a brand-name drug expires, other drug products usually seek market authorization. Chemically based drugs having small molecules, the alternatives are generic products; with biologically based drugs having large molecules, the alternative products are biosimilars. This chapter considers the design and analysis of studies for the assessment of interchangeability of biological products. It discusses criteria for drug interchangeability in terms of population and individual bioequivalence for generic drug products. The chapter also presents a newly proposed scaled criterion for drug interchangeability that could be applied to both generics and biosimilars. It then summarizes various study designs for assessing the risk of switching, alternating, and both switching and alternating. A unified approach for assessing drug interchangeability of biosimilar products is also provided. Based on the totality biosimilarity index for the assessment of biosimilarity, the switching index and alternating index for addressing drug interchangeability of biosimilar products can be obtained under an appropriate switching design and alternating design, respectively.