This introduction describes fundamental differences and assumptions between conventional drug products and follow-on biologics. It presents scientific factors and practical issues that are commonly encountered in the development of biosimilar products. The chapter focuses on issues surrounding biosimilars, including manufacturing, quality control, clinical efficacy, side-effects (safety), and immunogenicity. It attempts to address the challenges in imposing regulations that deal with these issues. For the comparison between drug products, some criteria for the assessment of bioequivalence, similarity and consistency are available in either regulatory guidelines/guidances and/or the literature. The assessment of biosimilarity between a biosimilar and the reference requires the establishment of the Fundamental Biosimilarity Assumption in order to bridge the surrogate endpoints and/or biomarker data to clinical safety and efficacy. Several numerical studies could be pursued, including simulations, meta-analysis, and/or sensitivity analysis to provide a better understanding of these product-specific guidelines/guidances and check the validity of the established Fundamental Biosimilarity Assumption, which is the legal basis for assessing biosimilarity.