What molecular attributes are responsible for the biological activity of a protein product? What changes in the molecule may affect its biological activity, stability, or immunogenicity? What structural components determine the half-life and biodistribution of the protein? When in the development of a product do we need to address these questions and what tools should be used? How are structure-function studies used in the evaluation of comparability and biosimilarity? Understanding how the structure of a molecule impacts its function and clinical profile is at the core of controlling protein manufacture and ensuring consistent clinical safety and efficacy throughout the product lifecycle. In this talk, we will discuss the regulatory expectations for evaluating structure and function in protein products, addressing what, how, when, and why to do these studies.