Scientific and regulatory issues around approval of follow-on protein products, referred to as biosimilars in Europe, have been a topic of great interest and debate recently. The central issue is our limited understanding of how the different quality attributes of a product have an impact on its safety and efficacy. Crucial gaps in our knowledge include a lack of standardization in the way in which data are collected, analyzed and reported, and limitations in the ability of non-clinical tools for predicting clinical safety and efficacy. Complexity of protein products with respect to the numerous quality attributes and complexity of the biotechnology processes and the raw materials add to the challenges. In this paper, recommendations are presented to help at least partially alleviate these challenges.