This chapter provides the description of X-ray crystallography as applied to protein structure. It discusses on what it can provide, and what its limitations are. The results of the crystallographic studies of antibodies, their fragments, and ligand complexes are analyzed, and it is described in the chapter, how the results of these studies have led to comparisons and generalizations with regard to the structure and biological properties of these molecules. Crystal structures for four intact antibodies are now reported in the literature. These structures are for the human myeloma proteins Dob, Mcg, and Kol, and for the murine monoclonal antibody MAb231, which is directed against canine lymphoma. The detailed structure of antibodies and various fragments, including isolated domains, from different isotypes and from different species has become available from X-ray crystallography. This chapter aims to review what is currently known about antibody structure from X-ray crystallography. A very detailed analysis of the structure of antibodies and antibody-antigen complexes are given.