Over the past two decades, advances in infrared instrumentation have allowed us to identify a vast and previously unseen population of low-temperature stars, brown dwarfs and free-floating extrasolar planets, collectively called ultracool dwarfs. These sources, with surface temperatures reaching below 0ºC, encompass three new spectral classes and include some of the nearest systems to the Sun. Research in this field is now concentrating on the physical characterization of the ultracool dwarf population and application to Galactic studies. In this talk, I will summarize the recent observational advances in ultracool dwarf research, including the recent discovery of the Y dwarf spectral class. I will then describe our ongoing IRTF/SpeX survey, which has measured the low-resolution, near-infrared spectra of over 1500 late M, L and T dwarfs and uncovered new subpopulations of young (5-30 Myr) brown dwarf, metal-poor halo brown dwarfs and short-period spectral binaries.