OBJECTIVES: To identify the types and quantities of evidence the FDA is requiring for biosimilar approval, and the corresponding evidence manufacturers have provided in their 351(k) biosimilar approval applications. METHODS: We searched through FDA general guidances, drug-specific approval documents, and webcasts to collect data. The following data were collected: approval indications and dates, reference product names, manufacturer names, evidence on similarity in analytical and functional characteristics, pharmacokinetics, pharmacodynamics, efficacy, safety, and immunogenicity, evidence used for extrapolation claims, and other notable points of discussion during FDA Advisory Committee meetings. RESULTS: There are currently four biosimilars approved in the United States. For these biosimilars, a large evidence base is provided to demonstrate similarity in analytical and functional characteristics, while fewer clinical studies are provided to demonstrate efficacy. Extrapolation allows biosimilars to be approved for indications in which clinical studies were not conducted, as long as they show similar analytical and functional characteristics relevant to the known or likely mechanism of action for the extapolated indication(s). CONCLUSIONS: As increasingly more biosimilars go through the 351(k) biosimilar approval pathway, we will better understand what evidence is required for biosimilar approval in the United States.