Most probabilistic seismic-hazard assessments require an estimate of Mmax, the magnitude (M) of the largest earthquake that is thought possible within a specified area. In seismically active areas such as some plate boundaries, large earthquakes occur frequently enough that Mmax might have been observed directly during the historical period. In less active regions like most of the Central and Eastern United States and adjacent Canada, large earthquakes are much less frequent and generally Mmax must be estimated indirectly. The indirect-estimation methods are many, their results vary widely, and opinions differ as to which methods are valid. This lack of consensus about Mmax estimation increases the uncertainty of hazard assessments for planned nuclear power reactors and increases design and construction costs. Accordingly, the U.S. Geological Survey and the U.S. Nuclear Regulatory Commission held an open workshop on Mmax estimation in the Central and Eastern United States and adjacent Canada. The workshop was held on Monday and Tuesday, September 8 and 9, 2008, at the U.S. Geological Survey offices in Golden, Colorado. Thirty-five people attended. The workshop goals were to reach consensus on one or more of: (1) the relative merits of the various methods of Mmax estimation, (2) which methods are invalid, (3) which methods are promising but not yet ready for use, and (4) what research is needed to reach consensus on the values and relative importance of the individual estimation methods.