The Monsoon '90 multidisciplinary field campaign was conducted over the U.S. Department of Agriculture's Agricultural Research Service Walnut Gulch experimental watershed in southeastern Arizona during June--September 1990. A primary objective of this combined ground, aircraft, and satellite campaign was to assess the feasibility of utilizing remotely sensed data coupled with water and energy balance modeling for large-area estimates of fluxes in semiarid rangelands. The experimental period encompassed a variety of vegetation, soil moisture, and rainfall conditions characterized by large temporal and spatial gradients. This preface outlines experimental objectives, briefly discusses the field campaigns, summarizes initial observations, and provides an overview of articles that are a part of the Monsoon '90 special section.