Recent studies have suggested a critical role for mitochondria in glutamate-mediated excitotoxic neuronal injury. Mitochondria in glutamate-stimulated neurons generate reactive oxygen species (ROS), accumulate substantial quantities of calcium, and typically depolarize. Moreover, preventing mitochondrial calcium accumulation protects neurons from excitotoxic injury. These studies establish several potential functions of mitochondria that could be involved in neuronal injury, but it remains unclear which of these phenomena are actually responsible for killing neurons. In addition, there are many aspects of normal mitochondrial function in neurons that are poorly understood. This presentation will illustrate some recent studies from this laboratory that provide insight into both the normal and abnormal function of neuronal mitochondria, and will focus on the characteristics of mitochondrial calcium transport, mitochondrial ROS generation, and a recently observed phenomenon of spontaneous mitochondrial depolarization.