Nanowires are one of the most promising candidates for new applications in nano-electronics, such as single electron transistors, optoelectronic devices and sensors. These nanowires are grown by the vapour-liquid-solid (VLS) mechanism. During VLS growth material supplied from the vapor phase diffuses through a liquid eutectic and is deposited on a solid surface. If the eutectic is in the form of a droplet, the deposit forms a pillar or "nanowire". By this method, single crystal wires with diameters down to five nanometers can be obtained. In close collaboration with Philips Research Laboratories we investigate the growth of InP and GaP nanowires to gather insight in the crystal growth mechanism, polymorphism and defect formation. Observations obtained by transmission electron microscopy are interpreted using morphological and 2D nucleation models. We recently showed that we can control the crystal structure of the nanowires and induce twinning superlattices by careful control of the growth conditions and by adding the proper dopants.