Silicon nanowires can be successfully grown by applying the vapor ? liquid ? solid process. In the case of the commonly used chemical vapor deposition technique, a Si containing gas/precursor is cracked at Au droplets acting as seeds. Si adatoms are subsequently dissolved in the liquid metal. Due to a supersaturation within this droplet, Si precipitates predominantly at the liquid ? solid interface ? a nanowire grows. A different situation occurs if nanowires are grown by molecular beam epitaxy via the vapor? liquid ? solid mechanism. The difference consists, for example, of the role of the metal seed, the morphology of the nanowires and their aspect ratio. In particular, surface diffusion including the metal used as well as Si, strongly influences the growth process. This article describes molecular beam epitaxy growth experiments of Si nanowires under ultra-high vacuum conditions and compares the results with other growth techniques.