This article will focus on the moisture induced degradation of the bulk adhesive when the degradation is permanent. This is a mechanism known as hydrolysis. It is dependent on the chemical nature of the polymer and the environmental conditions that are present during service. Moisture is the substance that causes the greatest difficulties in terms of environmental stability for many bonded or sealed joints. Water can be an exceptional problem because it is very polar and permeates most polymers. Other common fluids, such as lubricants and fuels, are of low or zero polarity and are not as likely to permeate and weaken adhesive or sealant joints. Moisture can alter the properties of the bulk material by changing its glass transition temperature, inducing cracks, or by chemically reacting with the polymer -- a process called hydrolysis. But before these mechanisms occur, the moisture must first find its way into the bulk polymer.