The field of nanomedicine involves the design and fabrication of novel nanocarriers for the intracellular delivery of therapeuticcargo or for use in molecular diagnostics. Although traditionally recognized for their ability to invade and infect host cells,viruses and bacteriophages have been engineered over the past decade as highly promising molecular platforms for thetargeted delivery and treatment of many human diseases. Inherently biodegradable, the outer capsids of viruses arecomposed entirely of protein building blocks, which can be genetically or chemically engineered with molecular imagingreagents, targeting ligands and therapeutic molecules. While there are several examples of viruses as in vitro molecular cargocarriers, their potential for applications in nanomedicine has only recently emerged. Here we highlight recent developmentstowards the design and engineering of viruses for the treatment of cancer, bacterial infections and immune system-relateddiseases.