Semiconductor nanowires are quasi-one-dimensional single-crystals that have emerged as promising materials for the development of photonic and electronic devices with enhanced performance. Nanowires offer solutions to some of the current challenges in science and engineering, but realization of their full potential will be ultimately dictated by the ability to control their structure, composition, and size with high accuracy. In this talk, I will discuss our recent results on the controlled growth, doping, and applications of III-V nanowires, as well as advanced electron microscopy techniques for direct correlation of structural and physical properties with high spatial resolution. We have developed a simple, yet powerful, approach to modulate both the diameter and composition of individual III-V nitride nanowires by adjusting in-situ the nanowire seed particle composition and volume. By elucidating the underlying mechanisms controlling structural evolution, we demonstrated the synthesis of axial InN/InGaN nanowire heterojunctions, compositionally tunable AlGaAs nanowires, GaAs/AlGaAs core-shell nanowires, and their controlled doping. We have demonstrated that the cathodoluminescence (CL) technique, coupled with scanning transmission electron microscopy (STEM), effectively bypasses the resolution limit of conventional far-field photoluminescence spectroscopy and allows direct structure-property correlation on the nanoscale. The CL-STEM optical studies of single nanowire heterostructures with spatial resolution of less than 20 nm will be discussed. Finally, applications of semiconductor nanowires for LED and solar cell applications will be described.