Field studies were conducted to evaluate the performance of sampling methods for measuring the coarse fraction of PM(sub 10)(PM(sub 10-2.5)) in ambient air. In the first stage of this evaluation, both time-integrated filter-based and direct continuous methods were evaluated. As the primary basis of comparison upon which to evaluate the candidate coarse fraction sampling methods, the PM(sub 10-2.5) concentration was estimated by taking the difference between concentrations measured with a PM(sub 10) FRM sampler and a PM(sub 2.5) FRM sampler. Sampling sites in Gary, IN, Phoenix, AZ, and Riverside, CA were selected to provide diverse challenges to the samplers with respect to aerosol concentration, size distribution, and composition. The first performance evaluations were conducted during 2003 and 2004. Instrument manufacturers were provided the results of these studies and encouraged to revise their PM(sub 10-2.5) samplers to address the measurement uncertainties identified. EPA conducted a follow-on field study of these second-generation samplers in Phoenix, AZ during April and May, 2005. The 2005 field study also evaluated four new prototype samplers designed to measure ambient PM(sub 10-2.5) aerosols. Two of these newer samplers were time-integrated, filter-based designs, and two were continuous-measurement designs. Despite operational problems encountered with some of the instruments during this phase of the testing, overall measurement results were encouraging and the instrument manufacturers are continuing to improve the reliability of the coarse mode samplers.