Technical Abstract: The United States Department of Agriculture - Agricultural Research Service Walnut Gulch Experimental Watershed (WGEW) near Tombstone, Arizona, USA, has served as an outdoor laboratory for the study of semiarid hydrology, erosion and sedimentation since the 1950s. Hydraulic structures along the large watershed channels, a dense raingage network, and intensively monitored watersheds have yielded a unique database for analyzing and interpreting rainfall, runoff, and erosion processes on semiarid lands. The monitoring program has evolved in response to changes in measurement technology as well as to changes in research scope. The research program at the WGEW has been expanded to include meteorologic, soil chemistry, remote sensing, soil moisture, and carbon flux measurements. In celebration of the 50th anniversary of the WGEW, The First Interagency Conference on Research in the Watersheds (ICRW) was held in Benson, Arizona in October of 2003. The conference was a forum for information exchange among researchers working at the watershed scale and stakeholders working on watersheds. This paper describes instrumentation and monitoring on the WGEW beginning with a historical perspective of the ARS watershed research network and site selection followed by an overview of the challenges associated with instrumenting a semiarid watershed. Initial challenges associated with establishing the core rainfall and runoff networks, the evolution of instrumentation, and current data management challenges are described. In addition, a summary of the ICRW meeting is presented.