Semiconductor nanowires combine the material properties of semiconductors, which are ubiquitous in modern technology, with nanoscale dimensions and as such, are firmly poised at the forefront of nanotechnology research. The rich physics of semiconductor nanowire optics, in particular, arises from the increased interaction between light and matter that occurs when light is confined to dimensions below the size of its wavelength, in other words, when the nanowire serves as a light trapping optical cavity, which itself is also a source of light. Light confinement is taken to new extremes by coupling to the surface plasmon modes of metallic nanostructures, where light acquires mixed photonic and electronic character, and which may focus light to deep-subwavelength regions amenable to the dimensions of the electron wave. This thesis examines how the integration of "plasmonic optical cavities" and semiconductor nanowires leads to substantial modification (and enhancement) of the optical properties of the same, resulting in orders-of-magnitude faster and more efficient light emission with colors that may be tuned as a function of optical cavity geometry. Furthermore, this method is applied to nanowires composed of both direct and indirect bandgap semiconductor materials resulting in applications such as light emission from high-energy states in light emitting materials, highly enhanced broadband light emission from nominally non-light emitting (dark) materials, and broadband (and anomalous) enhancement of light absorption in various materials, all the while maintaining the unifying theme of employing integrated plasmonic-semiconductor optical cavities to achieve tailored optical properties. We begin with a review of the electromagnetic properties of optical cavities, surface plasmon-enhanced light emission in semiconductors, and the key physical properties of semiconductor nanowires. It goes without saying that this thesis work resides at the interface between optical physics and materials science.