Dr. Greenwald has conducted experimental physics research on the Alcator A, C, and C-Mod tokamaks, including studies of energy and particle transport, pellet fueling, density limits, neutral particle measurements, and studies of energetic ion dynamics. He was part of the design team for Alcator C-Mod. His recent work has focused on the role of critical gradients in determining plasma temperature profiles, on the EDA H-mode regime, and on the role of turbulent transport in determining the tokamak density limit. This latter work is aimed toward defining a "first principals" theory for the limit with the long term goal of obtaining reliable predictions and extrapolations to future machines. Dr. Greenwald's work in pellet fueling on Alcator C included the discovery of a new regime of enhanced energy and particle confinement. This was the first attainment of an internal transport barrier in a tokamak which is recognized as one of two generic approaches (along with H-Mode) to improving tokamak confinement. Because of the correlation with peaked density profile, the work spurred interest in the hi or Ion Thermal Gradient driven (ITG) mode, now believed to be the principal mechanism for anomalous transport in tokamaks.