The Fire Regime Groups layer characterizes the presumed historical fire regimes within landscapes based on interactions between vegetation dynamics, fire spread, fire effects, and spatial context (Hann and others 2004). Fire regime group definitions have been altered from previous applications (Hann & Bunnell 2001; Schmidt and others 2002; Wildland Fire Communicator's Guide) to best approximate the definitions outlined in the Interagency FRCC Guidebook. These definitions were refined to create discrete, mutually exclusive criteria. This layer was created by linking the LANDFIRE Biophysical Settings (BpS) layer to the Fire Regime Group rulesets. This geospatial product should display a reasonable approximation of Fire Regime Group, as documented in the Refresh Model Tracker. The Historical Fire Regime Groups data layer categorizes simulated mean fire return intervals and fire severities into five fire regimes defined in the Interagency Fire Regime Condition Class Guidebook. The classes are defined as follows: Fire Regime I: 0 to 35 year frequency, low to mixed severity Fire Regime II: 0 to 35 year frequency, replacement severity Fire Regime III: 35 to 200 year frequency, low to mixed severity Fire Regime IV: 35 to 200 year frequency, replacement severity Fire Regime V: 200+ year frequency, any severity Additional data layer values were included to represent Water (111), Snow / Ice (112), Barren (131), and Sparsely Vegetated (132). Vegetated areas that never burned during the simulations were included in the category "Indeterminate Fire Regime Characteristics" (133); these vegetation types either had no defined fire behavior or had extremely low probabilities of fire ignition.