This field guide is a forest management tool for field identification of biotic and abiotic agents that damage native trees in Colorado, Kansas, Nebraska, South Dakota, and Wyoming, which constitute the USDA Forest Service's Rocky Mountain Region. The guide focuses only on tree diseases and forest insects that have significant economic, ecological, and/ or aesthetic impacts; this guide does not necessarily cover all possible damage agents. Management suggestions are provided where available. The field guide is divided into two sections: one describes both diseases and damage caused by animals and abiotic factors, and the other describes insects. Agents are presented by the type and/or location of the injury on the tree. Written descriptions, color photographs, a general index, an index by host tree species, descriptive tables, and line drawings are all provided to assist users in identifying damaging agents.