Dendrimers are a very special type of hyperbranched polymers, synthesized step by step from a central core, by the repetition of a sequence of reactions. Phosphorus-containing dendrimers, that are dendrimers possessing one phosphorus atom at each branching point, possess numerous properties, most generally due to the presence of multiple functional groups in close proximity, which may induce cooperative effects. Such effect is in particular highly desirable when creating sensors. Indeed, in order to comply with environmental friendly criteria, a sensor must be sensitive, specific, and reusable. The use of phosphorus-containing dendrimers for creating sensors may afford such properties. This review is divided in 4 main headings. The first one concerns the self-diagnosis of the dendritic structure, purity, size and properties; the second one describes the use of these dendrimers as chemical sensors; the third one emphasizes the role of dendrimers for the elaboration of biological sensing devices; and finally the fourth one describes the use of fluorescent dendrimers for biological imaging. Several of these properties are due to the presence of phosphorus at all branching point of these structures.