Pioneering research by Alex L. Shigo and his associates has produced a series of pictorial guidelines to provide a better understanding of how trees respond to wounding and subsequent microbial infections that lead to wood decay. The purpose of this paper is to visually summarize through the use of 96 color photographs and illustrations, the varied patterns of wood discoloration and decay observed in the dissection of thousands of trees. This information has served as a conceptual framework for understanding the biochemical processes that limit the spread of wood-destroying infections initiated by wounding during the maturation of all trees. This understanding has helped those who work with forests, trees, and wood to resolve practical problems and improve the health and productivity of trees and forests.