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 restoration since water can degrade dental restorations. A hydrophilic adhesive would attract more water into the adhesive layer and would have faster degradation. Therefore, the ideal adhesive should be hydrophilic before polymerization and hydrophobic after polymerization. Adhesive functional monomers in general are acidic, but some are more hydrophilic than others, which can increase the permeability of the adhesive layer, even after polymerization. For example, MDP and BPDM are the most hydrophobic monomers, while GPDM and Penta-P are more acidic and hydrophilic.