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Kentucky bluegrass (Poa pratensis) has been increasing in the northern Great Plains.  Current estimates are that 75 percent of the range sites in the North Dakota and a majority of range sites in South Dakota and Nebraska have some degree of Kentucky bluegrass encroachment.  Data from the Northern Great Plains Research Laboratory (USDA-ARS) as well as other anecdotal and published reports suggest that Kentucky bluegrass encroachment has occurred within the last 30 years.  There have been numerous suggested causes for the increase in Kentucky bluegrass during this time period.  Some of these include 1) increased precipitation during the 1990s; 2) increases in available nitrogen either through the atmosphere or through nitrogen fixation; 3) adherence to later turn-on dates for grazing; 4) use of deferment and rest as a management tool; 4) use of the take half leave half utilization measurement and 5) alterations in the fire regime.  Each of these drivers could by itself or in concert with other drivers, helped to promote the encroachment of Kentucky bluegrass.  Most of the past efforts have focused on the control of Kentucky bluegrass without a clear understanding of the drivers. However, understanding the drivers of the encroachment will help develop new control strategies and enhance existing control strategies.