Explains how seedlings grow into mature trees and how stands of trees develop. Describes basic tree growth and growth processes (photosynthesis, respiration, and translocation-assimilation). Discusses how thinning affects individual tree growth and stand growth, including growth responses, thinning shock, tolerance levels of common Pacific Northwest tree species, crown classification systems, and stand yields. Recommends six objectives for a good thinning plan and provides suggestions on timing, thinning intensity, species and tree selection, and logging methods. Includes illustrations of the kinds of wood that make up a tree, cross-sections of trees properly and improperly thinned, and the relationship of crown class to tree height and age.