Via the FAA:
RVSM was implemented to reduce the vertical separation above flight level (FL) 290 from 2000-ft minimum to 1000-ft minimum. It allows aircraft to safely fly more optimum profiles, gain fuel savings and increase airspace capacity. The process of safely changing the separation standard required a study to assess the actual performance of airspace users under the old vertical separation standard of 2000-ft and potential performance under the new standard 1000-ft. In 1988, the ICAO Review of General Concept of Separation Panel (RGCSP) completed this study and concluded that safe implementation of the 1000-ft separation standard was technically feasible. RVSM was subsequently implemented and today RVSM represents a global standard for 1000-ft vertical separation
The entire western hemisphere implemented RVSM FL290—FL410 on January 20, 2005.