Studying the impacts of volcanic eruptions on climate is important because it helps us improve climate models, it allows us to make seasonal and interannual climate forecasts following large eruptions, it provides support for nuclear winter theory, and it allows us to separate the natural causes of interdecadal climate change from anthropogenic effects, giving us greater confidence in the attribution of recent global warming to anthropogenic causes. While much has been learned since the large 1991 eruption of Mt. Pinatubo in the Philippines, there are still quite a few outstanding research problems, which are discussed here. These questions include: What exactly goes into the atmosphere during an explosive eruption? How can we better quantify the record of past climatically-significant volcanism? Can we design an improved system for measuring and monitoring the atmospheric gases and aerosols resulting from future eruptions? How can we better model the climatic impact of eruptions, including microphysics, chemistry, transport, radiation, and dynamical responses? How do high-latitude eruptions affect climate? How important are indirect effects of volcanic emissions on clouds? Where are the important potential sites for future eruptions?.