Casting a wide net, this volume provides personal and professional information on some 445 American and Canadian naturalists and environmentalists, who lived from the late 15th century to the late 20th century. It includes explorers who published works on the natural history of North America, conservationists, ecologists, environmentalists, wildlife management specialists, park planners, national park administrators, zoologists, botanists, natural historians, geographers, geologists, academics, museum scientists and administrators, military personnel, travellers, government officials, political figures and writers and artists concerned with the environment. Some of the subjects are well known. The accomplishments of others are little known.

Each entry contains a succinct but careful evaluation of the subject's career and contributions. Entries also include up-to-date bibliographies and information concerning manuscript sources.