We review the current developments of DNA-based nanostructures for drug delivery, immunotherapy, diagnostics and molecular biology. DNA is a powerful building block, which by the nature of predictable base pairing, allows the creation of molecular scaffolds, cages and multifunctional carriers with nanoscale dimensions. These engineered constructs have unsurpassed structural qualities such as full control over size, shape and dispersity. Site-specific surface modification enables the presentation of biomolecules at defined distances and stochiometries, which allows tailored cell targeting and substance delivery on demand. As the first successful in vivo applications of DNA nanostructures have recently been demonstrated, we now expect a burst of biomedical studies involving this rapidly progressing technology.