Arid and semiarid environments are important, comprising more than one third of the earth’s land surface. Most of the Western and Southwestern United States is arid and drought conditions are becoming more prevalent here and globally. This course will cover the assessment of ecological risk over multiple scales of biological organization for plants and animals in arid and semiarid environments. Such assessments are required to support cleanup decisions for sites contaminated with hazardous or radioactive wastes. The evaluation of ecological effects in these environments needs to be technically defensible – a challenging proposal if standard risk assessment approaches are used. For example, most terrestrial toxicity bioassays are primarily applicable to mesic sites. This half-day workshop provides an overview of the tools and challenges for performing ecological assessments in arid and semiarid ecosystems. To bring a full understanding of ecological effects characterization to the attendees, this course attempts to achieve several major goals: namely, illustrate conceptual site models and problem formulation development through biotic transport in the desert and bioaccumulation in arid-adapted species; describe how contamination effects in semiarid soils can be interpreted using standard toxicity tests and using site-specific biota; consider special needs and management of aquatic resources in arid environments; and document the strengths and limitations of using arid ecosystem ecological risk assessments to support informed environmental decision making in Southwestern and Western states. This course is specifically designed to present ways for new practitioners to succeed in developing scientifically defensible ecological risk assessments.