Six mercury continuous emission monitoring (CEM) systems provided by two leading mercury (Hg) CEM system manufacturers were tested at five coal combustion utilities. The linearity, response time, day-to-day stability, efficiency of the Ho speciation modules, and case of use were evaluated by following procedures specified in the Code of Federal Regulation Title 40 Part 75 (40 CFR Part 75). Mercury monitoring results from Hg CEM systems were compared to an EPA-recognized reference method. A sorbent trap sampling system was also evaluated in this study to compare the relative accuracy to the reference method as well as to Hg CEM systems. A conceptual protocol proposed by U.S. EPA (Method 30A) for using an H- CEM system as the reference method for the Hug relative accuracy (RA) test was also followed to evaluate the workability of the protocol. This paper discusses the operational experience obtained from these field studies and the remaining challenges to overcome while using Hg CEM systems and the sorbent trap method for continuous Flu emission monitoring.