Adhesion is a universal phenomenon, daily touching the lives of individuals. It is virtually impossible to scan an environment and not note any number of materials or products that develop their usefulness as a result of bonding or coating in some fashion. Yet in spite of this ubiquitous nature, surprisingly little is known about the primary causes of adhesion. This chapter briefly reviews the fundamental theories of adhesion and focuses on one of them, direct chemical bonding, as it relates to wood substrates.