<p>Multiphase flow dynamics are integral to many propulsion, sprays, energetics, and industrial processes. Practical systems, especially in combustion, typically involve multidimensional spatial structures and complex and coupled physics interactions. At some operating conditions, flow mixing, combustion chemical reactions, and flow residence time scales are relatively similar and therefore coupled (i.e., each affects the other). For example, the combustion and atomization of liquid fuel govern the performance of combustors. In addition to spray-air interactions, injection strategies may rely on spray-wall interactions to achieve improved mixing and performance. Understanding and predicting these flows requires advanced experimental diagnostics that provide information on local state variables with high spatiotemporal resolution. However, multiphase flow dynamics integral to these combustion systems may not be fully resolved with conventional one or two-dimensional diagnostics. Tomographic reconstructions yield 3D spatial information and may provide high-fidelity data to fill the technology gap. Performing these 3D diagnostics with adequate time-resolution is necessary to capture the full dynamics of high-speed flows. This work focuses on developing, applying, and evaluating non-intrusive 4D (x,y,z,t) volumetric imaging in challenging combustion environments. Each optical diagnostic approach probes a different phase of combustion experiments in a non-instructive manner. For example, Schlieren imaging visualizes the index of refraction gradients corresponding to density changes in the gas phase. This work uses various optical approaches (e.g., scattering, Schlieren, or fluorescence) with 4D imaging to provide quantitative measurements of different combustion phenomena. Parallel ray-tracing simulations are utilized to guide diagnostic development and quantify measurement capabilities. This work presents significant high-speed diagnostic improvements for combustion applications relevant to defense, energy generation, and propulsion.</p>