The spatial domain definition, usually in the form of a mesh, plays an important role in many codes solving systems of PDEs on massively parallel systems. In addition to defining the spatial domain, the mesh is also the basis for partitioning these problems for parallel solution, and is the vehicle for solution data during post-processing. I will discuss the MOAB mesh library and its use in these various contexts. Related libraries for CAD geometry (CGM) and mesh-geometry relations (Lasso) will also be described.