A new standard setting method, Mapmark, was recently developed by ACT Inc. in the course of a contract with the National Assessment Governing Board (NAGB) to set achievement levels for the 2005 National Assessment of Educational Progress (NAEP) in Grade 12 mathematics. Mapmark includes elements of the bookmark method (Lewis, Mitzel, & Green, 1996; Mitzel, Lewis, Patz, & Green, 2001), item maps (Masters, Adams, & Loken, 1994), and expected percent correct scores on clusters of assessment items representing areas of knowledge, skills, and abilities called "domains" (Schulz, Lee, & Mullen, 2005). The present paper provides a detailed description of the Mapmark method, as implemented in the Achievement Level Setting (ALS) meeting conducted for NAGB, and presents results bearing on the procedural validity of the method. Procedural validity is assessed through panelists' responses to process evaluation questionnaires and by other data collected in the meeting. It is concluded that the Mapmark process exhibits evidence of procedural validity. (Contains 13 tables and 15 figures.)