Type Ia supernovae (SNe Ia) are one of the most effective observational tools for measuring the expansion history of the Universe. Their successful use in cosmology is due to the discovery of empirical relations that dramatically decrease the dispersion in peak luminosities at optical wavelengths.The first of these is the well-known correlation of absolute magnitude with light curve shape, and the second is a strong dependence of peak luminosity on color that is in the same sense as dust reddening, but with an average value of the ratio of total-to-selective extinction, Rv, that is significantly less than what is typically observed for interstellar dust in the Milky Way. Recent work indicates that there is an additional dependence of both Rv and the optical luminosity of SNe Ia on the mass (and, presumably, the metallicity) of the host galaxies.

These complications can largely be avoided by working in the near-infrared, where dust extinction corrections are much smaller than in the optical, and where the luminosity-light curve shape dependence can also largely be ignored. The combination of optical + near-infrared photometry shows that SNe Ia have Rv values that span the range from ~2-3 that may be a function of the amount of reddening itself. The application of individual values of Rv rather than a global average leads to significant improvements in the dispersion of the Hubble diagram, both at optical and near-IR wavelengths. Further improvements should be possible through near-infrared spectroscopy since K-corrections are still only approximately understood at these wavelengths. Near-infrared spectroscopy also offers a new tool for probing the progenitors and explosion physics of SNe Ia.