Anisotropic noble metal nanocrystals are promising novel nanomaterials because of their excellent catalytic and optical properties. In particular, the nanowire structure prepared by an aggregation method acts as a nanocatalyst with high catalytic activity owing to the disordered state of metal atoms within the aggregated domains. To prepare these nanowires, it is necessary to adequately understand their formation dynamics and mechanisms. In this chapter, we describe the nanowire formation dynamics and mechanisms involving the aggregation of nanocrystals, selective adsorption of a capping agent, and their catalytic and optical properties. We also discuss our recent research works on Pd nanowires, dendritic Au nanowires, and ultrathin straight Au–Ag nanowires. These results are useful for preparing functional nanowires with high catalytic activity and unique optical properties.