Among the different types of dendrimers, those that incorporate phosphorus in various positions in their structures present fascinating properties and have wide applications in pharmaceutical fields such as drug delivery (including gene transfection), diagnosis and imaging. Therapeutic agents can be encapsulated within the dendritic architecture, or chemically attached or physically adsorbed onto the dendrimer surface. Alternatively, phosphorus dendrimers can be developed as drugs themselves, for instance to treat cancer, inflammations, infections and neurodegenerative diseases, although there are fewer examples of this second approach. Here, we review the most relevant in vivo biological activities of phosphorus dendrimers (whether as active drugs per se or as nanocarriers) across various therapeutic domains, including anti-inflammation, transmissible spongiform encephalopathies, ocular hypertension, infection, gene therapy, ultrasound-targeted microbubble destruction and two-photon in vivo imaging. We discuss the success of phosphorus dendrimers in nanomedicine and look ahead to future innovations in the field.