Nanocomposite thermites, or Metastable Intermolecular Composites (MICs), are energetic systems involving the reaction between nanoparticles of a metal fuel and another metal or metal oxide. When nanoparticles are used, the interfacial contact area and homogeneity of mixing are greatly improved, dramatically decreasing the characteristic mass diffusion length between the fuel and the oxidizer. Nano-sized aluminum is commonly used as a fuel, due to a combination of its abundance, good reactivity, and its ability to produce environmentally benign reaction products. A variety of oxidizers have been studied depending on the particular application. Nanocomposite thermites are currently being investigated for uses in propellants, pyrotechnics, and explosives, as well as some more exotic applications such as micro-propulsion and joining applications. Despite the research efforts and potential applications, the reaction mechanism remains poorly understood. As the particle size transitions into the nanometer regime, properties such as the melting temperature, surface energy, drag force, along with the characteristic time scales of thermo-chemical processes can change. In an exothermically reacting system, all of these considerations must be taken into account simultaneously, a rather daunting task. However, if we design parametric experiments to look at relative trends, we can develop scaling laws and determine which parameters are perhaps the most important in the reaction mechanism. This work largely involves combusting thermite materials in a pressure cell, and also uses new techniques such as inducing a reaction inside an electron microscope with a specially designed heating holder. The results suggest that the pressurization and optical emission can arise from fundamentally different phenomena. A reactive sintering mechanism occurs which rapidly decomposes the oxidizer and pressurizes the system. This is followed by the remainder of the fuel burning in a gaseous, pressurized environment, where the burning rate is controlled by the fuel. Also in this work, we combust new fuels and oxidizers such as nano-sized boron, AgIO ₃, and Ag₂O. Boron can be used as an additive to increase the energy density in thermites. The silver-based oxidizers are currently being investigated in nanocomposite thermites for their ability to generate a product which can effectively destroy harmful biological spores, such as Anthrax.