Objective: The objective of this study was to assess and compare the requirements in the guidelines published by EMA, FDA and TMMDA (Turkish Medicines and Medical Devices Agency) on biosimilars’ registration. Methods: Guidelines officially published by EMA, FDA and TMMDA were evaluated, and comparison was made in aspects namely classification and legal status of the guidelines, selection of the reference product and extrapolation of the clinical data between different indications. Results: EMA has 3 main guideline groups (overarching guidelines, product specific guidelines and other guidelines relevant for biosimilars) consisting of 15 guidelines in total, FDA has 8 guidelines regarding formal meetings between FDA and sponsors, demonstration of biosimilarity, quality considerations, clinical pharmacology data, reference product exclusivity, Q&A regarding implementation of the biologics price competition and innovation act of 2009, demonstration of interchangeability, and TMMDA has 1 guideline. In terms of legal status, EMA and TMMDA guidelines are legally binding in contrast to FDA. Regarding selection of reference product; EMA requires the reference product to be previously approved by EC, FDA requires the reference product to be previously approved by FDA (otherwise the reason should be justified), TMMDA requires the reference product to be approved by an authority. In terms of extrapolation of the clinical data between different indications, all the authorities requires applicants to provide sufficient scientific justification for extrapolating clinical data to support a determination of biosimilarity for each indication. Conclusion: Despite several differences, the main requirements stated by EMA, FDA and TMMDA were found to be highly similar. Basically, biosimilarity is expected to be justified by demonstrating that there are no major differences in terms of quality, efficacy and safety in non-clinical and clinical issues between biosimilar drug and reference drug as a result of comparative in vivo and in vitro studies.