L and T dwarfs emit most of their radiation in the near infrared and their spectral energy distributions are dominated by strong molecular absorption bands. These highly structured energy distributions lead to JHK magnitudes that are extremely sensitive to the exact filter bandpass used. In the case of the T dwarfs, the differences between commonly used photometric systems can be as large as 0.4 mag at J and 0.5 mag at J-K. To address this problem, we have synthesized J,H and K magnitudes for

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... me of the common photometric systems and present transformation equations. If the spectral type of the dwarf is known, our transformations allow data to be converted between systems to 0.01 mag, which is better than the typical measurement uncertainty. Transforming on the basis of color alone is more difficult because of the degeneracy and intrinsic scatter in near-infrared colors of L and T dwarfs; in this case J magnitudes can only be transformed to < 0.05 mag and H and K to < 0.02 mag.