Solar flares and coronal mass ejections are the most energetic events in the solar system. Their energy derives from the solar magnetic field, which is rapidly released via a process called magnetic reconnection. Reconnection occurs at microscopic length scales when magnetic fields break and change topology, allowing the macroscopic system to reach a lower energy state. These solar eruptions can contribute to space weather effects at the Earth, such as knocking out satellite communication, disrupting air travel, and causing health risks to astronauts. Magnetic reconnection is at the heart of solar eruptions and space weather-relevant physics in the Earth’s magnetosphere, as well as related phenomena in other Sun-like stars and compact objects, other planetary magnetospheres, and in fusion devices. Its effects are the subject of a number of upcoming satellite missions in both solar and magnetospheric contexts. In this presentation, a number of theoretical and computational results will be discussed. A model for supra-arcade downflows in solar flares will be presented. This model raises questions about the physics of localized vs. extended three-dimensional reconnection, so their differences were studied. New results on the location of reconnection on the dayside of the Earth’s magnetosphere and its observational signatures will be discussed. In addition, a newly discovered parameter regime of fast reconnection will be shown. In all cases, observational consequences will be stressed, including indirectly determining coronal conditions during solar flares, identifying reconnection events in the solar wind, and determining the importance of reconnection in planetary magnetospheres.