Much of my research focuses on the theory of physical processes in the interstellar medium, the diffuse gas between the stars. How do stars form out of this tenuous gas, and what effect do the stars have back on the interstellar medium? Jerry Ostriker and I developed the three-phase model of the interstellar medium, which has been widely used to organize and interpret observations of the diffuse interstellar medium. With my colleagues and students, I have worked on the theory of the evaporation of clouds by both hot gas and ionizing radiation, the evolution of supernova remnants and stellar wind bubbles, the structure and emission spectrum of interstellar shocks, the evolution of interstellar dust grains, the structure of molecular clouds, and on the theory of active galactic nuclei, particularly the study of reverberation mapping and of Compton-heated coronae and winds above accretion disks. I am currently concentrating on the theory of star formation, including the formation of the first stars. My students and I use both analytic and numerical techniques to address these problems. Richard Klein and I have established the Berkeley Astrophysical Fluid Dynamics Group to develop the technique of adaptive mesh refinement for numerical simulations of astrophysical fluid dynamics, particularly star formation.