We present K- and L-band photometry obtained with the Keck I telescope for a representative sample of L and T dwarfs. These observations were motivated in part by the dominant role H2O and CH4 play in shaping the flux near 2 and 3 μm and by the potential use of these bands as indicators of spectral class in the infrared. In addition, these observations aid the determination of the bolometric luminosity of L and T dwarfs. Here we report the K, L', and Ls magnitudes of our objects and the trends observed in the (K-L') and (K-Ls) colors as a function of L and T dwarf spectral class. We compare these colors with theoretical models, derive a relationship between effective temperature and L spectral class, and compare our temperature estimates with others.