The presence of nitric acid clouds near the tropical tropopause was proposed over a decade ago, although little has been learned about them since. Particle extinction measurements at three wavelengths from the Halogen Occultation Experiment offer compelling evidence for the existence of nitric acid clouds near the tropical tropopause. Most of these clouds are consistent with nitric acid trihydrate, and a small fraction can be explained by liquid ternary H2SO4-H2O-HNO3 aerosols.