We are conducting a joint theoretical/experimental research program on hydraulic fracturing. Newly developed two-dimensional numerical models (which include complete descriptions of the elastic continuum and porous flow fluids) have been applied to analyze the effects of pore pressure on the fracturing process. By means of small-scale experiments, we are acquiring a better understanding of the effects of the in-situ stress field, the porosity and permeability of the solid, and the presence of interfaces or layering in the solid. Experimentally, we have been studying the growth of cracks near an interface in several materials, including polymethylmethacrylate (PMMA), Nugget sandstone, and Indiana limestone. Results have shown that the mechanical properties of the interface relative to the properties of the materials on either side are important. A crack will not cross a well-bonded interface between two pieces of PMMA, even in the presence of a 13.79-MPa (2000-psi) normal load. Cracks will cross a well-bonded interface from PMMA to limestone, but not vice versa. Similarly, cracks will propagate across a bonded interface from Nugget sandstone to limestone, but not the other way. Pressure-driven cracks will cross an unbonded interface between limestone blocks at normal loads as low as 3.45 MPa (500 psi).