Nanometre-scale electronic structures are of both fundamental and technological interest: they provide a link between molecular and solid state physics, and have the potential to reach far higher device densities than is possible with conventional semiconductor technology(1.2). Examples of such structures include quantum dots, which can function as single-electron transistors(3,4) (although their sensitivity to individual stray charges might make them unsuitable for large-scale devices) and semiconducting carbon nanotubes several hundred nanometres in length, which have been used to create a held-effect transistor(5). Much smaller devices could be made by joining two nanotubes or nanowires to create, for example, metal-semiconductor junctions, in which the junction area would be about 1 nm(2) for single-walled carbon nanotubes. Electrical measurements of nanotube ＇mats＇ have shown the behaviour expected for a metal-semiconductor junction(6). However, proposed nanotube junction structures(7) have not been explicitly observed, nor have methods been developed to prepare them. Here we report controlled, catalytic growth of metal-semiconductor junctions between carbon nanotubes and silicon nano,vires, and show that these junctions exhibit reproducible rectifying behaviour.