Biological drugs make up one of the fastest-growing sectors of the pharmaceutical and biotechnology industry. This chapter provides a comprehensive review of regulatory requirements, criteria for biosimilarity, and statistical methods, including power calculations for determining the sample size needed for the assessment of biosimilarity between a proposed biosimilar product and an innovative biologic product. In addition, it introduces a unified approach for demonstrating biosimilarity using the concept of reproducibility. The chapter also describes several criteria for the primary assessment of bioequivalence or similarity proposed by the FDA, discussing their applicability to the assessment of biosimilarity. The biosimilarity index has the advantages that it is robust with respect to the selected study endpoint, biosimilarity criteria, and study design and the probability of reproducibility will reflect the sensitivity of heterogeneity in variance. The chapter also evaluates the biosimilarity index proposed by Chow et al. for the assessment of the (average) biosimilarity between the innovator and reference products.