When an innovator biological product is going off patient, biopharmaceutical/biotech companies may file applications for regulatory approval of follow-on biologics (or biosimilars). Unlike the small molecular drug products, the complexity and heterogeneity of the molecular structure, complicated manufacturing process, different analytical methods, and possibility of severe immunogenicity reactions of biological products have made the evaluation of biosimilarity between follow-on biologics and the innovator product a great challenge for both scientific community and regulatory agencies. In this presentation, an overview of scientific factors for assessment of biosimilarity and drug interchangeability as discussed at a recent FDA Public Hearing on Approval Pathway for Biosimilar and Interchangeable Biological Products held in Silver Spring, Maryland, between November 2-3, 2010 will be reviewed. In addition, a newly developed biosimilar index based on the concept of reproducibility probability of claiming biosimilarity is proposed. The proposed biosimilar index can be used to address the questions that (1) how similar is similar? and (2) the issue of switching and alternating for drug interchangeability under an appropriate study design.