In the effort to reduce the scientific and technical uncertainties over regulation of airborne particulate matter in the United States, Research Priorities for Airborne Particulate Matter: II. Evaluating Research Progress and Updating the Portfolio, the second book in a four-part series requested by Congress, describes the plans of the committee to monitor the progress of the research on particulate matter conducted by the U.S. Environmental Protection Agency (EPA), other federal and state government agencies, and nongovernmental organizations. The book also reviews and updates the committee's portfolio of recommended research in its first volume, Research Priorities for Airborne Particulate Matter: I. Immediate Priorities and a Long-Range Research Portfolio (NRC, 1998). The committee substantially revised two of the ten high-priority research areas recommended in Part I. Part II notes that Congress, EPA, and the scientific community have given strong support to the committee's recommendations and have implemented substantial changes in research efforts in response to Part I of the series. One important research area-studies of the effects of long-term exposure to particulate matter and other major air pollutants-however, does not appear to be underway or planned.