A diabody molecule comprising a first polypeptide chain and a second polypeptide chain, the polypeptide chains being covalently bonded to one another, wherein: I. the first polypeptide chain comprises: (i) a first domain comprising a binding region of a light chain variable domain of a first immunoglobulin specific for an epitope, (ii) a second domain comprising a binding region of a heavy chain variable domain of a second immunoglobulin specific for an epitope, and (iii) a third domain comprising: an Fc domain; or a hinge domain; or a light chain constant region; which first domain and second domain are covalently linked such that the first domain and second domain do not associate together to form an epitope binding site or an epitope binding site; II. the second polypeptide chain comprises: (i) a fourth domain comprising a binding region of a light chain variable domain of the second immunoglobulin specific for the epitope, (ii) a fifth domain comprising a binding region of a heavy chain variable domain of the first immunoglobulin specific for the epitope, and (iii) a sixth domain comprising: an Fc domain, where the third domain comprises an Fc domain; or a peptide portion comprising the amino acid sequence FNRGEC (SEQ ID NO: 23), where the third domain comprises an Fc domain, or a hinge domain; or a heavy chain constant region 1, a hinge region, a heavy chain constant region 2 and a heavy chain constant region 3, where the third domain comprises a light chain constant region; which fourth domain and fifth domain are covalently linked such that the fourth domain and fifth domain do not associate together to form an epitope binding site or an epitope binding site; wherein the first domain and the fifth domain associate together to form a binding site that binds the epitope; wherein the second domain and the fourth domain associate together to form a binding site that binds the epitope; and wherein: the second and the third domains; and/or the fifth and the sixth domains; are linked to one another by an intervening polypeptide linker consisting of a sequence that is selected from the group consisting of SEQ ID NOs.:278, 280, 281, 283, 284, 286, 287, 288, 289, 290, 291, 293, 295 and 297. I. the first polypeptide chain comprises: (i) a first domain comprising a binding region of a light chain variable domain of a first immunoglobulin specific for an epitope, (ii) a second domain comprising a binding region of a heavy chain variable domain of a second immunoglobulin specific for an epitope, and (iii) a third domain comprising: an Fc domain; or a hinge domain; or a light chain constant region; which first domain and second domain are covalently linked such that the first domain and second domain do not associate together to form an epitope binding site or an epitope binding site; an Fc domain; or a hinge domain; or a light chain constant region; II. the second polypeptide chain comprises: (i) a fourth domain comprising a binding region of a light chain variable domain of the second immunoglobulin specific for the epitope, (ii) a fifth domain comprising a binding region of a heavy chain variable domain of the first immunoglobulin specific for the epitope, and (iii) a sixth domain comprising: an Fc domain, where the third domain comprises an Fc domain; or a peptide portion comprising the amino acid sequence FNRGEC (SEQ ID NO: 23), where the third domain comprises an Fc domain, or a hinge domain; or a heavy chain constant region 1, a hinge region, a heavy chain constant region 2 and a heavy chain constant region 3, where the third domain comprises a light chain constant region; which fourth domain and fifth domain are covalently linked such that the fourth domain and fifth domain do not associate together to form an epitope binding site or an epitope binding site; an Fc domain, where the third domain comprises an Fc domain; or a peptide portion comprising the amino acid sequence FNRGEC (SEQ ID NO: 23), where the third domain comprises an Fc domain, or a hinge domain; or a heavy chain constant region 1, a hinge region, a heavy chain constant region 2 and a heavy chain constant region 3, where the third domain comprises a light chain constant region; the second and the third domains; and/or the fifth and the sixth domains;