Optically thin dust emission at submm and mm wavelengths is a powerful probe of the density and temperature structure of the envelope of protostars. Radiative transfer models of the dust continuum emission constrain theoretical predictions of the structure of forming protostellar cores. The resolution of current submm and mm bolometer arrays, such as SCUBA, image the outer envelope on scales of 500 to 30,000 AU, while current mm interferometric observations probe the emission on scales down to 10 AU. The basic procedures for understanding the density and temperature structure towards low-mass (e.g., solar mass) star-forming cores are reviewed and important systematic effects are discussed.