Ultraluminous infrared galaxies (ULIRGs), which emit quasar-like luminosities in the far-infrared part of the spectrum, are exceedingly rare in the local Universe - less than 3% of the glaxies in the IRAS Bright Galaxy Sample have luminosities above 10^12 Lsun and fall into this class. While the environments and properties of ULIRGs are well understood - they nearly always are interacting/merging, spiral galaxies with large, centrally concentrated reservoirs of dense molecular gas - the energy sources which heat the dust are still under debate. If galaxies were dusty at high redshifts, ULIRGs may provide us with a valuable analog to the process of galaxy building and star formation at early epochs - much of which may have thus far eluded detection at optical/UV wavelengths. SIRTF will allow us to make great strides in our understanding of ULIRGs, by (1) increasing the number of known objects over a large range in redshift, and (2) allowing a detailed study of the excitation sources via mid-infrared fine-structure and PAH emission features. We will review the most recent ground and space-based data on ULIRGs, and assess the prospects for infrared photometry and spectroscopy with SIRTF.