I am a theoretical astrophysicist. Over the years, I have worked on the dynamics of globular clusters and galactic nuclei, interstellar scintillation, gamma-ray bursts, strong astrophysical shocks, star formation near quasars, planetesimal formation, angular-momentum transport in astronomical accretion disks, tidal effects in stars and gaseous planets, gravitational microlensing, and laboratory astrophysics. At present, I am concentrating on accretion and evaporation of protoplanetary disks, and some dynamical questions concerning exoplanets. I spend summers consulting for various government agencies on scientific and technical matters relating to national security.