ParFlow is a parallel, three-dimensional, variably saturated groundwater flow code that is especially suitable for large scale, high resolution problems ParFlow's development and application has been on-going for more than 10 years and resulted in some of the most advanced numerical solvers and multigrid preconditioners for massively parallel computer environments that are available today. Many of the numerical tools developed within the ParFlow platform have been turned into libraries that are now distributed and maintained at LLNL ( Hypre and SUNDIALS, for example). ParFlow has recently been extended to coupled surface-subsurface flow to enable the simulation of hillslope runoff and channel routing in a truly integrated fashion. An additional major advantage of ParFlow is the use of a sophisticated octree-space partitioning algorithm to depict complex structures in three-space, such as topography, different hydrologic facies, and watershed boundaries. All these components implemented into ParFlow enable large scale, high resolution watershed simulations. ParFlow has also been fully-coupled to a land surface model (CLM) and to a mesoscale atmospheric model (ARPS).