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The continued integration of smaller, higher-functioning devices in the Aerospace, Defense & Space sectors is making it more challenging than ever to minimize assembly failure, especially where reliability is top priority and fail-safe processes, and materials are the standard. Assembly failures due to poor adhesive bonding tend to linger causing excessive manufacturing downtime, scrap, costly rework, and delays. This paper introduces the science behind adhesion. Regardless of the application, the market or the adhesive technology being used, such as epoxy, silicone, urethane, paste or film, achieving a proper assembly bond is critical for application success. Adhesion is achieved via two mechanisms, mechanical and chemical bonding. To optimize this the adhesive must flow (or wet) across the substrate. This maximizes the interaction between the adhesive and substrate allowing flow into microscopic substrate pores for enhanced mechanical bonding as well as interaction between adhesive and substrate for chemical bonding. The molecular force of attraction between an adhesive and the substrate is determined by the surface energy of the substrates. The substrate surface energy influences the ability of the adhesive to flow and wet the substrate impacting adhesion. Measuring substrate water contact angle is an easy method to determine the relative surface energy thereby gaining information about the pending adhesive bond. This paper reviews these key factors for adhesion and presents results of an application study aimed at understanding the sensitivity and relationship of substrate water contact angle versus substrate cleanliness. The goal is to be able to use substrate water contact angle measurements as a predictive tool for adhesion.