The formation of nanostructures with controlled size and morphology has been the focus of intensive research in recent years(1-10). Such nanostructures are important in the development of nanoscale devices and in the exploitation of the properties of nanomaterials(9). Here we show how tree-like nanostructures ('nanotrees') can be formed in a highly controlled way. The process involves the self-assembled growth of semiconductor nanowires via the vapour-liquid-solid(11) growth mode. This bottom-up method uses initial seeding by catalytic nanoparticles(12) to form the trunk, followed by the sequential seeding of branching structures. Each level of branching is controlled in terms of branch length, diameter and number, as well as chemical composition. We show, by high-resolution transmission electron microscopy, that the branching mechanism gives continuous crystalline (monolithic) structures throughout the extended and complex tree-like structures. The controlled seeding method that we report here has potential as a generic means of forming complex branching structures, and may also offer opportunities for applications, such as the mimicking of photosynthesis in nanotrees.