Ideally, a dentin bonding agent should have both hydrophilic and hydrophobic ends. The hydrophilic end displaces the dentinal fluid to wet the surface. The hydrophobic end bonds to the composite resin. Bonding: The forces or energies between atoms or molecules at an interface that hold two phases together. Mechanism of bonding: Dentin adhesive molecule has a bifunctional structure: M-R-X Where, M is the double bond of methacrylate which copolymerizes with composite resin. R is the spacer which makes the molecule large. X is a functional group for bonding, which bonds to an inorganic or organic portion of dentin. Ref: Textbook of Operative Dentistry, Nisha and Amit Garg, 3rd edition, Pg no:239