The present invention relates to amino acid sequences that are directed against Interleukin-23 (IL-23). The amino acid sequences of the present invention comprise two NANOBODIES® against IL-23 and one NANOBODY® against serum albumin, linked by two linkers (9GS linkers). In particular, the invention relates to the amino acid sequences of SEQ ID NO: 2 and SEQ ID NO: 3 (listed in Table 1 and FIG. 1) (also referred to herein as “anti-IL 23 polypeptides of the invention”).