What happens as light traverses a medium composed of both traditional materials and many (105-1012 cm-3) nanoparticles? These types of systems are present in many active areas of research in the nanotechnology sphere. Examples include nanoparticles in aqueous and non-aqueous solvents during chemical synthesis or for solar energy harvesting applictions; nanoparticles embedded in homogeneous and non-homogeneous solids for photocatalysis; nanoparticles in biological tissue for medical appplications, and more. Because nanoparticles composed of a certain material can have optical properties very different from the bulk material, these types of systems also display unique optical properties. In this thesis I outline an approach to solving light transport in nanomaterial systems based on the Monte-Carlo method. This method is shown to be optimal for nanomaterial systems where the extinction coefficient is composed of relatively equal contributions of scattering and absorption.Furthermore, I show that this computational tool can be utilized to solve problems in a wide variety of fields. In plasmonic photocatalysis, where mixtures of nanoparticles are driven resonantly to efficiently catalyze chemical reactions, this method elucidates the photothermal contribution. Experimental results combined with calculations suggest that the photocatalysis of a novel antenna-reactor complex composed of an Al core and a Cu2O shell is primarily from hot-electron injection. Calculations involving taking optical images of objects through mixtures of nanoparticles explain the phenomenon that absorptive particles can enhance image quality and resolution of images taken through a scattering medium. Previous reports on this effect were limited in their explanation. We show that the reduced scattering coefficient is not sufficient to explain the phenomenon. Rather, all of the optical parameters must be known independently. The addition of absorptive particles increases image quality be selectively removing photons with the longest path-length through the system. These photons are the most likely to cause image distortion, having undergone multiple scattering events, having lost the original information of the image.Simulations of light transport through highly concentrationed solutions of nanoshells (1109-11011 NP/ml) show a localization and efficiency of absorbed light that explains previous results obtained in light-triggered release of DNA from nanoparticle surfaces. The strong temperature gradients obtained from these calculations help clarify previous results, which showed DNA release below the dehybridization temperature with CW laser irradiation. Further studies motivated by these calculations elucidate two regimes in light-triggered release with NIR radiation. CW radiation causes dehybridization of DNA due to melting, whereas ultrafast radiation causes Au-S bond breakage. Although previous studies have shown Au-S bond breakage for 400 nm ultrafast irradiation, this work is the first to explicitlyAdvisors/Committee Members: Halas, Naomi J (advisor).