Callen directs the university's Center for Plasma Theory and Computation. He is heavily involved in the U.S. fusion program, and has served on a number of Department of Energy review panels and fusion plasma physics journal editorial boards. His current plasma research includes developing fluid/kinetic hybrid descriptions of magnetically confined plasmas; studying the macroscopic phenomenology caused by MHD tearing-type modes and their effects on plasma confinement in tokamaks; models for turbulence and plasma transport in tokamak, stellarator and reversed-field pinch plasmas; attempts to correlate the theory of collective macroscopic instabilities, which may give the ultimate limit on the plasma pressure and current that can be magnetically contained, with experimental results from tokamaks, stellarators, and reversed-field pinches. Professor Callen is a fusion plasma theoretician with research interests in developing and applying plasma theory and computation to present plasma confinement experiments, and fusion reactor design studies.