Continuous emission monitoring systems (CEMS) for mercury (Hg) are receiving increased attention and focus. Their potential use as a compliance assurance tool is of particular interest. While Hg CEMs are currently used in Europe for compliance purposes, use of Hg CEMS in the United States (U.S.) has focused on combustion research and Hg control technology evaluation applications. Hg CEMS are now receiving increased attention as compliance assurance tools. Several programs exist to evaluate Hg CEM measurement performance. It is through these efforts that application-specific measurement issues are investigated. Collectively, these efforts have served to advance the state-of-the-art of the technology as evidenced by the number and types of CEMS now available and the various applications in use.