The measure of airborne particulate matter PM10 is effectively defined for European regulatory purposes by the European Committee for Standardisation (CEN) standard EN 12341:1998--a gravimetric filter-based method. However, compared with the reference methods written for other pollutants, specifically to address the needs of the Air Quality Directives, EN 12341 falls short in areas such as ongoing Quality Assurance and Quality Control. Also, scientific knowledge about PM has moved on considerably since 1998, so that we now realize that implementations of variations allowed within EN 12341:1998 can lead to substantial differences in results obtained. These issues were partially addressed within the CEN standard for PM2.5, EN 14907:2005, which, for example, included tighter control of the handling of sampled filters to reduce losses of semi-volatile material. CEN Technical Committee 264 Working Group 15 is currently revising EN 12341, and has the opportunity to improve the reference method further, for example by tighter specification of the filter material, or by changing the specification for the humidity at which the filters are brought to equilibrium. Any changes must, however, be considered in the context of the regulatory implications of the standard, so that improvements in scientific quality must be balanced with the effect of a step change in reference method, and with associated costs. The paper outlines the present position of the EN 12341 revision and recent changes to EU Air Quality legislation.