Abstract
The Observatorio Astronómico Nacional at San Pedro Mártir is located at 31°02'40''N and 115°28'00''W on the summit of the Sierra San Pedro Mártir on the Baja California peninsula, Mexico, at 2800 m above sea level. The results of more than three decades of site characterization work are summarized. These cover the following aspects: weather, cloud coverage, atmospheric optical extinction, night sky brightness, millimetric and infrared opacity and seeing. Overall, San Pedro Mártir is one the most favorable accessible sites in the Northern Hemisphere for astronomical observations. The number of cloudless nights, minimal turbulence and favorable local wind conditions, to mention but a few of its characteristics, make it well-suited for large telescopes. Long-term monitoring of the site is being undertaken by the Universidad Nacional Autónoma de México and other international institutions.
