The Douglas-fir tussock moth, Orgyia pseudotsugata McD., periodically defoliates Douglas-fir, true firs, and other host trees in forests of the western United States. In the Northern Region, these infestations occur about once every decade. This history covers the earliest recorded outbreak in northeastern Washington from 1928 to 1930 and includes information about outbreaks in northern Idaho and eastern Washington from 1944 to 1947, northeastern Washington and northern Idaho from 1950 to 1955, northern Idaho and western Montana from 1961 to 1965, and the current outbreak in northern Idaho, northeastern Washington, and western Montana which began in 1970. These outbreaks usually last from 2 to 4 years in forested areas and typically go through a buildup phase the first year, an outbreak phase the second year, and a declining phase the third year due to parasites and/or polyhedrosis virus. Exceptions are: It may take an outbreak 2 years to build up or occasionally natural control agents can cause a population collapse at the end of the second year.