This chapter discusses protein therapeutics including recombinant proteins and monoclonal antibodies. It provides examples of recombinant proteins approved for human use to treat disorders like hemophilia, diabetes, and cystic fibrosis. The chapter outlines different expression systems used to produce recombinant proteins, including bacteria, yeast, insect, and mammalian cells. It also describes the structure of antibodies and the development of monoclonal antibodies as therapeutic agents, from mouse antibodies to humanized antibodies to reduce immunogenicity.