The practical realization of nanoscale electronics faces two major challenges: the precise engineering of the building blocks and their assembly into functional circuits. In spite of the exceptional electronic properties of carbon nanotubes, only basic demonstration devices have been realized that require time-consuming processes. This is mainly due to a lack of selective growth and reliable assembly processes for nanotubes. However, graphene offers an attractive alternative. Here we report the patterning of graphene nanoribbons and bent junctions with nanometre-precision, well-defined widths and predetermined crystallographic orientations, allowing us to fully engineer their electronic structure using scanning tunnelling microscope lithography. The atomic structure and electronic properties of the ribbons have been investigated by scanning tunnelling microscopy and tunnelling spectroscopy measurements. Opening of confinement gaps up to 0.5 eV, enabling room-temperature operation of graphene nanoribbon-based devices, is reported. This method avoids the difficulties of assembling nanoscale components and may prove useful in the realization of complete integrated circuits, operating as room-temperature ballistic electronic devices.