The role of phosphorhydrazide derivatives, in particular of type RP(X)(NMeNH2)2 for the synthesis of macrocycles and of type Cl2P(X)NMeNH2 (X = O, S in both cases) for the synthesis of dendrimers, will be shown in this review. In both cases, condensation reactions with different types of aldehydes (often aromatic) afford phosphorhydrazone derivatives, which are the building blocks for these special architectures. After the synthetic aspects, selected properties related to catalysis, nanomaterials, and biology/nanomedicine will be displayed.