A diabody complex capable of binding serum albumin, wherein said diabody complex comprises three polypeptide chains each comprising an N-terminus and a C-terminus, wherein: said first and second polypeptide chains are covalently bonded to one another via a disulfide bond, and each comprise, in the N-terminal to C-terminal direction, a light chain variable domain linked to a heavy chain variable domain, wherein: said light chain variable domain of said first polypeptide chain associates with said heavy chain variable domain of said second polypeptide chain to form a binding site specific for a first epitope; and said light chain variable domain of said second polypeptide chain associates with said heavy chain variable domain of said first polypeptide chain to form a binding site specific for a second epitope; said first polypeptide chain comprises a heterodimer formation driving polypeptide portion, said heterodimer formation driving polypeptide portion being positioned C-terminal to said heavy chain variable domain of said first polypeptide chain, and being an E-coil or a K-coil; wherein said E-coil is 4 heptameric repeats of EVAALEK comprising the amino acid sequence of SEQ ID NO: 299, and said K-coil is 4 heptameric repeats of KVAALKE comprising the amino acid sequence of SEQ ID NO: 300; and said third polypeptide chain comprises a polypeptide portion of a protein capable of binding to said serum albumin, and a heterodimer formation driving polypeptide portion that is: an E-coil comprising the amino acid sequence of SEQ ID NO: 299 if said first polypeptide chain comprises a K-coil comprising the amino acid sequence of SEQ ID NO: 300, or a K-coil comprising the amino acid sequence of SEQ ID NO: 300 if said first polypeptide chain comprises an E-coil comprising the amino acid sequence of SEQ ID NO: 299; wherein said heterodimer formation driving polypeptide portion of said first and third polypeptide chains are complexed together to thereby form said diabody complex, and said diabody complex is capable of simultaneously binding to said first epitope, to said second epitope and to said serum albumin. said first and second polypeptide chains are covalently bonded to one another via a disulfide bond, and each comprise, in the N-terminal to C-terminal direction, a light chain variable domain linked to a heavy chain variable domain, wherein: said light chain variable domain of said first polypeptide chain associates with said heavy chain variable domain of said second polypeptide chain to form a binding site specific for a first epitope; and said light chain variable domain of said second polypeptide chain associates with said heavy chain variable domain of said first polypeptide chain to form a binding site specific for a second epitope; said light chain variable domain of said first polypeptide chain associates with said heavy chain variable domain of said second polypeptide chain to form a binding site specific for a first epitope; and said light chain variable domain of said second polypeptide chain associates with said heavy chain variable domain of said first polypeptide chain to form a binding site specific for a second epitope; said first polypeptide chain comprises a heterodimer formation driving polypeptide portion, said heterodimer formation driving polypeptide portion being positioned C-terminal to said heavy chain variable domain of said first polypeptide chain, and being an E-coil or a K-coil; wherein said E-coil is 4 heptameric repeats of EVAALEK comprising the amino acid sequence of SEQ ID NO: 299, and said K-coil is 4 heptameric repeats of KVAALKE comprising the amino acid sequence of SEQ ID NO: 300; and said third polypeptide chain comprises a polypeptide portion of a protein capable of binding to said serum albumin, and a heterodimer formation driving polypeptide portion that is: an E-coil comprising the amino acid sequence of SEQ ID NO: 299 if said first polypeptide chain comprises a K-coil comprising the amino acid sequence of SEQ ID NO: 300, or a K-coil comprising the amino acid sequence of SEQ ID NO: 300 if said first polypeptide chain comprises an E-coil comprising the amino acid sequence of SEQ ID NO: 299; an E-coil comprising the amino acid sequence of SEQ ID NO: 299 if said first polypeptide chain comprises a K-coil comprising the amino acid sequence of SEQ ID NO: 300, or a K-coil comprising the amino acid sequence of SEQ ID NO: 300 if said first polypeptide chain comprises an E-coil comprising the amino acid sequence of SEQ ID NO: 299;