Hydrophilic means having a strong affinity to water, while hydrophobic means having little or no affinity to water. Dentin is hydrophilic and contains water. A dental adhesive should be hydrophilic before polymerization in order to penetrate into the dentin structure to have great wetting ability and maximum mechanical bonding. However, water is the enemy of dental adhesives after restorationsince water can degrade dental restorations. A hydrophilic adhesive would attract more water into the adhesive layerand would have faster degradation. Therefore, theideal adhesive should be hydrophilic before polymerization and hydrophobic after polymerization. Adhesive functionalmonomers in general are acidic, but some are more hydrophilic than others, which can increase the permeabilityof the adhesive layer, even after polymerization. For example, MDP andBPDM are the most hydrophobic monomers, while GPDM and Penta-P are more acidic and hydrophilic.