The concept of biosimilarity emerged in the early 2000s, when the patents of innovative biotechnological medicinal products expired and/or when they started losing data protection, opening the door to the application for the marketing authorization of putative biological generics. With the notable exception of a European product-specific guideline on low-molecular-weight heparins, virtually all guidance documents are focusing on recombinant proteins. The quality development, particularly the characterization and quality comparability exercise, appear to be the starting point for the stepwise development of biosimilars. This chapter addresses the biosimilars that contain a recombinant protein as an active substance, focusing on their quality development. Different types of biological assays can be used such as enzyme activity, ligand or receptor binding assays, and cell-based assays. Immunological functions of antibodies and antibody-based products should be investigated in terms of specificity, affinity, binding kinetics, and Fc functional activity.