Culturally modified trees, or CMTs, are a phenomenon of forest-dwelling peoples worldwide, from North America to Scandinavia, to Turkey, to Australia. Living trees from which materials are harvested (edible inner bark, pitch and resin, bark, branches), or which are modified through coppicing and pollarding to produce wood of a certain size and quality, or which are marked in some way for purposes of art, ceremony, or to indicate boundary lines or trails, all represent the potential of sustainable use and management of trees and forested regions. Often their use is associated with particular belief systems or approaches to other life forms that result in conservation of standing trees and forests, and preserving or enhancing their habitat value and productivity, even while they serve as resources for people. Various types of culturally modified trees have religious or spiritual significance, tying people to their ancestors who used the trees before them, and signifying traditional use and occupancy of a given region. Although some CMTs are legally protected to some extent in some jurisdictions, many are at risk from industrial forestry, urban expansion and clearing land for agriculture, and immense numbers of CMTs from past centuries and decades have already been destroyed. The diverse types, and the patterns of CMT creation and use, need further study; these trees, collectively, are an important part of our human heritage.