A common phenomenon of yellowjacket (Vespula) population dynamics is the occurence of “wasp years” or years of extreme abundance contrasted with other years of extremely low populations. The number of flying workers per square meter aids in defining wasp outbreaks in the Pacific Northwest. In addition to this estimate, public reaction and colony collections provide qualitative evidence for establishing 1973, 1977, and 1979 as outbreak years ofV. vulgarisorV. pensylvanicaor both throughout most of this region. Spring (April to June) weather conditions during the critical stage of colony founding appear to be the major factor determining outbreak populations. Wasp years have not occurred in the Pacific Northwest since 1971 unless this spring period was warm and dry. Several meteorological factors during this April-through-June period from 1973 through 1979 substantiate this hypothesis. Although these weather factors are possibly the most important ones determining wasp years, they are not necessarily the factors regulating wasp cycles; however, they may prove to be valuable tools in predicting yellowjacket outbreaks in the Pacific Northwest.