The Department's tradition in stellar and galactic dynamics follows pioneering work by Lyman Spitzer on the dynamical evolution of globular clusters, and by Martin Schwarzschild on collisionless equilibria of galaxies. Ostriker now studies the dynamics context of cosmology and galaxy formation. David Spergel's recent work in galactic structure has focused on using tidal streams to probe the lumpiness of the Galactic halo. He has also investigated high-velocity clouds, and the dynamics of galactic bars. Goodman has current interests in astrophysical fluid dynamics and MHD, especially disk accretion and stellar and planetary tides. Roman Rafikov is interested in planetary dynamics in relation to the formation of planetary systems, especially velocity evolution and particle growth in disks of planetesimals---the building blocks for Earth-like planets and for the cores of giant planets. He has also studied the stability of galactic disks and the dynamics of planets embedded in gaseous disks. Scott Tremaine is interested in a wide range of issues in astrophysical dynamics, including the formation and evolution of planets, the long-term stability of planetary systems, small bodies in the solar system (comets, asteroids, the Kuiper belt, and planetary rings), debris disks and planetesimal disks, binary stars and stellar systems, structure and formation of galaxies, dynamics of dark matter, and black holes and galactic nuclei. Other astrophysical dynamicists in Princeton include Piet Hut of the Institute for Advanced Study (IAS), with interests in dense stellar systems, collisional N-body problems, and dynamical computation; and Edward Belbruno, formerly of Princeton's Program in Applied and Computational Mathematics, with interests in celestial mechanics, astrodynamics, chaos theory. Peter Goldreich, an emeritus professor at the IAS and a frequent visitor, has broad interests in astrophysics and planetary science.