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The Pacific Northwest, as considered here, consists of California north of San Francisco, Oregon, Washington, southern British Columbia, Idaho, and western Montana. In the context of the flora and fauna of western North America, the Pacific Northwest contains or is contiguous with four major biogeographic regions, namely, California, the Great Basin, the Rocky Mountains, and the Pacific Northwest. The Northwest region contains numerous mountain ranges, high desert, the Columbia River Basin, part of the Snake River, the Puget and the Willamette Valleys, and the northern Pacific coast.

The vegetation in the Pacific Northwest is very diverse and includes a flora adapted to coastal, desert, and alpine environments. The prevalent forest trees are species of conifers with Douglas-fir, ponderosa pine, lodgepole pine, and redwoods representing major forest types. Other conifers include spruce, hemlock, larch, true fir, cedar and numerous species of pine. The prevalent woodland trees include oak, alder, poplar and aspen, maple, and juniper. The understory vegetation in these forests and woodlands is also very rich in species. For example, some of the most prevalent species of flowering trees and shrubs occur in the genera: Acer, Alnus, Amelanchier, Arbutus, Arctostaphylos, Artemisia, Baccharis, Chrysolepis, Ceanothus, Celtis, Cercocarpus, Cornus, Corylus, Crataegus, Fraxinus, Gaultheria, Holodiscus, Juniperus, Lithocarpus, Myrica, Oemleria, Pachistima, Philadelphus, Physocarpus, Prunus, Populus, Purshia, Quercus, Rhamnus, Rhododendron, Ribes, Rubus, Salix, Sambucus, Sorbus, Spiraea, Symphoricarpos, Umbellularia, Vaccinium.

Approximately 180-200 species of butterflies and skippers and 400-500 species of micromoths are listed in the Pacific Northwest. However, the scientific effort that goes into understanding the Lepidoptera fauna is not evenly distributed, thus the low numbers of micromoths. The presence and identity of butterfly and skipper species in the Pacific Northwest is very well known, but the presence and identity of macromoths has been the subject of less attention, while the presence and identity of the micromoths is relatively little studied. The macromoth fauna of the Northwest has never been the subject of a comprehensive study and many species remain to be discovered and described. For instance, Mesogona rubra, Oncocnemis greyi, and Cerastis enigmatica are recently described species discovered and named within the last few years. When more studies are conducted we expect the butterfly species count will remain nearly the same, but the macromoth species count could increase another 25 percent, to around 1,500 species, and the micromoth species count is likely to equal or exceed the number of macromoths.

The literature related to the identification of these moths principally exists in technical scientific journals, if at all. Macromoth identification is facilitated by books such as A Field Guide to the Moths of Eastern North America (Covell 1984) and a series of publications under the title Moths of North America, published by the Wedge Foundation. However, no general guide to the macromoths of western North America exists.

Identifying field-collected macromoths, either adults or caterpillars, to the species level is essential to performing natural history observations, accurately labelling collections, and conducting detailed ecological studies on host plants, parasitoids, and using Lepidoptera as indicator species in assessing environmental impacts. This guide to identification of the adults of macromoths of forests and woodlands with an emphasis on the fauna of the Pacific Northwest serves to complement a field guide to the caterpillars of Pacific Northwest forests and woodlands (Miller 1995). We have selected 251 species for diagnostic narratives and photographs of adults. Also, we have included discussion on over 300 additional species in diagnosing similar species to those featured with photographs. The geographical range for these species as a whole covers not just the Pacific Northwest States but also west of the Rocky Mountains and from northern California to southern British Columbia.