The 1993 OH Tropospheric Photochemistry Experiment was performed in the forested Colorado mountains during late summer and early fall 1993. Measurements of the OH radical were made by four different techniques; one pair provided an extensive intercomparison data set. A large number of other species that control OH and HO2 concentrations were also determined. These ancillary variables have been useful both in understanding intercomparison issues and in testing detailed model predictions of OH photochemistry in this environment. The paper places this campaign in historical and scientific perspective; it discusses the results in terms of their contributions both to our ability to measure OH radicals and to our understanding of the pertinent tropospheric photochemistry; and it provides a view toward, and lessons for, future campaigns. We conclude that OH is indeed being measured and that it forms very useful tests of photochemical models but that we do not fully understand the underlying chemistry in this environment.¿ 1997 American Geophysical Union