A general synthetic method has been developed to control both the diameter and the length of nanowires during growth. This approach exploits monodisperse nanocluster catalysts to define both the nanowire diameter and the initiation of nanowire elongation during growth by a vapor-liquid-solid mechanism. To demonstrate this new approach, crystalline indium phosphide (InP) nanowires have been synthesized using a laser catalytic growth (LCG) process combined with gold nanocluster catalysts. InP nanowires with nearly monodisperse diameters of 10, 20, and 30 nm were grown from nanocluster catalysts having diameters of 10, 20, and 30 nm, respectively. High-resolution transmission electron microscopy studies show that the InP nanowires prepared in this manner are single crystals with a [111] growth direction. In addition, studies of nanowire growth as a function of growth time have shown that nanowire length is directly proportional to growth time and have enabled the preparation of InP nanowires with narrow length distributions centered at 2, 4, 6, and 9 mum. The new level of synthetic control afforded by our approach should enable better-defined fundamental studies of nanowires and open up new opportunities for the assembly of functional nanodevices.