On March 20, 1984 the U.S. Environmental Protection Agency proposed revisions to the national ambient air quality standards (NAAQS) for particulate matter. New primary standards were proposed for particulate matter measured as PM(sub 10) (particles with aerodynamic diameters equal to or less than a nominal 10 micrometers). In addition to the revisions to the NAAQS's, EPA also proposed a new Federal Reference Method (FRM) for the measurement of PM(sub 10) in the atmosphere and appropriate revisions to the EPA regulations concerning Ambient Air Monitoring Reference and Equivalent Methods and Ambient Air Quality Surveillance. The proposed FRM for PM(sub 10) is based on discrimination and selection of PM(sub 10) particles by inertial separation in a specially shaped inlet, followed by conventional filtration of a measured volume of sampled air and determination of the net weight gain of the filter. Wherever feasible, sampler requirements are prescribed by functional or performance specifications to allow maximum flexibility in the design or configuration of PM(sub 10) samplers. Candidate samplers would have to be tested according to explicit test procedures and formally designated as reference or equivalent methods. This paper will review the proposed NAAQ's and related monitoring requirements and discuss EPA's PM(sub 10) sampler development and evaluation activities.