As we move into a new age of elementary particle physics, defined by high intensity experiments and precision measurements, previously negligible effects can become significant and quantifiable. This talk will cover the theoretical and experimental projects I worked on throughout my PhD, with an overall focus on these effects on the physics of neutrinos at current and future experiments. In particular, I will first discuss a recent investigation into the seasonal variation of solar neutrino flux measured at the Super-Kamiokande experiment in Japan. Following that, I will present a phenomenological analysis of the matter effects on long-baseline neutrino oscillations that might be detected by the next-generation Hyper-Kamiokande experiment, which has recently been formally approved for seed funding and construction and is expected to replace Super-K in the coming decade. Finally, within the context of some more fundamental questions that cannot be directly measured by today's experiments, I will review the theoretical Littlest Seesaw model of the origin of neutrino masses and examine how it can still be studied and tested against current oscillation data.