Porous materials are common on the Earth's surface and in the crust. Accordingly, their physical properties have received a lot of attention. A century ago, Maxwell and Rayleigh each modeled the physical properties of aggregate materials as discrete spheres embedded in continuous matrices. Although the particles of interest in snow are not spheres and do have interconnections, these basic models give first-order predictions of the thermal conductivity. In the last 3 decades, scientists have attempted to make the predictions more precise by determining the effect of geometry on heat and mass flow using basic physical models and data collected from images of planar sections of aggregates. Under favorable circumstances, physical parameters and quantitative microscopic parameters of an aggregate may be highly correlated, but physical understandings of the geometric effects are not likely to arise from such studies until physical models can be based on measurable fundamental parameters. In snow, as in other aggregates, that goal seems to be a long way from realization. ¿ American Geophysical Union 1995