Nanowires are promising candidates for nanoscale electronic devices, and controlling nanowire properties is a major focus of research in nanotechnology. While there have been tremendous advances in both the understanding and the engineering of nanowire properties, the mechanism by which nearly all nanowires are grown (the 'vapor-liquid-solid' or VLS mechanism) is less understood. This is all the more remarkable given the fact that VLS growth has been studied for more than 40 years. We have investigated the growth of Si nanowires under conditions in which all the critical growth parameters are strictly controlled, while growth is directly observed using real-time in situ electron microscopy.@footnote 1@ We find that the migration of Au during nanowire growth places fundamental limits on the shape, length, and sidewall properties of the nanowires. Our results show that the conventional picture of VLS growth is insufficient when the growth takes place under conditions in which the substrate surface cleanliness, the deposition and formation of the Au-Si eutectic catalyst droplets, as well as the Si source gas purity and background contamination levels are rigorously monitored and controlled. Surprisingly, we find that under the cleanest growth conditions, VLS growth does not result in long wires of fixed diameter with clean sidewalls. Au from the catalyst droplets coats the nanowire sidewalls, terminating VLS growth when the catalyst is consumed. In addition migration of Au from one catalyst droplet to another during nanowire growth (catalyst droplet coarsening) determines the sidewall profile. In other words, the high surface diffusivity of Au, combined with the strong tendency of clean Si surfaces to getter Au atoms, together with the conventional VLS process, results in Au-coated nanowires of limited length and variable diameter. @FootnoteText@ @footnote 1@ J.B. Hannon, S. Kodambaka, F.M. Ross, and R.M. Tromp, Nature 440 (2006) 69.