Nanowires make possible to manipulate light in novel methods and thus are promising materials for advanced optoelectronics. To exploit the potential, the growth behavior has to be controlled since it dominates the physical and chemical states and, in turn, the optical properties of nanowires. In this chapter, the vapor-liquid-solid (VLS) mechanism for the growth and modulation of nanowires was discussed. The chapter first reviewed the fundamental aspects of the VLS mechanism. Then the state of the art of the growth and modulation of nanowires for optoelectronics were discussed from the point of view of the critical issues pertaining to this mechanism. Some examples of optoelectronic devices that had been fabricated based on the VLS mechanism were also reviewed in an effort to cover the cutting edge technology in this area. Lastly, a summary and several different perspectives on the VLS mechanism were presented.