Bioremediation is an emerging in situ treatment technology for soil and groundwater cleanup. Research in the past decade has made significant progress toward understanding how to stimulate microbial growth in the subsurface by optimizing the physical/chemical conditions. Recent laboratory observations and field demonstrations indicate that bioremediation can also be limited by mass transfer processes. In this paper, factors restricting microbial growth are reviewed, and the importance of bioavailability on the performance of in situ bioremediation is discussed by using aromatic hydrocarbons as model contaminants. Successful application of bioremediation relies upon an understanding of interactions among microorganisms, organic contaminants and soil/aquifer materials. Applications of biofilm kinetics toward this goal are addressed. Model simulations and laboratory studies suggest that both low temperature and slow desorption rate could greatly lengthen the time required for effective in situ bioremediation of aromatic hydrocarbons.