Bats represent the second most diverse group of mammals inhabiting the western slopes of the Cascade Range in southern Washington and Oregon Coast Range. Bat populations may well be sensitive to changes in forest age, structure, or distribution, but their nocturnal habits and high mobility render the study of the habitat requirements of bats problematical. Unlike most other groups of vertebrates, bats are difficult to either observe or capture, and survey methods are poorly known. The paper reviews techniques for surveying bat populations and presents the methodology used in the Old-Growth Forest Wildlife Habitat Program in the Pacific Northwest.