The Observatorio Astronómico Nacional at San Pedro Mártir is located 31°02'40'' N and 115°28'00'' W on the summit of the Sierra San Pedro Mártir in the Baja California peninsula, Mexico, at 2800 m above sea level. The results of nearly three decades of site characterization work are summarized. These cover the following aspects: weather, cloud coverage, local meteorology, atmospheric optical extinction, millimetric opacity, seeing, optical turbulence profiles, wind profiles and 3D simulations of atmospheric turbulence. Overall, San Pedro Mártir is one the most favorable sites in the world for astronomical observations. Its excellent turbulence and local wind conditions, to mention but two characteristics, make it particularly well suited for large telescopes. Long-term monitoring of the site is being undertaken by the National Autonomous University of Mexico and other international institutions.