The present invention relates to amino acid sequences (also referred to herein as “single variable domain(s) of the invention”. “Nanobody™ of the invention” or “Nanobodies™ of the invention”, ilpolypeptide(s) of the invention”) that are directed against (as defined herein) members of the Notch signalling pathway such as the four mammalian members of the Notch Receptors (Notch-1, Notch-2, Notch-3, Notch-4, hereinafter also “Notch Receptors”) and the five mammalian members of the Notch Ligands (the Delta-like family with “DLL1”, “DLL3”, “DLL4” wherein “DLL” stands for Delta-like ligand; the Jagged family with “Jagged-1”, “Jagged-2”, hereinafter also “Notch Ligands”), as well as to constructs that comprise or essentially consist of one or more such single variable domain(s) (also referred to herein as “construct of the invention” and “constructs of the invention”, “Nanobody™ of the invention” or “Nanobodies™ of the invention”, respectively). In particular, this invention relates to Notch pathway interfering single variable domains down- or up-regulating Notch signaling (also referred to herein “Notch-antagonists” or “Notch-agonists”).