The destructive potential of leucocytes needs to be fully held in check in the circulation, while being released at the site of tissue destruction and infection. Leucocyte adhesion molecules are the key to this regulation, the recognition by the leucocytes of areas of tissue damage, and to transendothelial migration of leucocytes into these areas. The three classes of leucocyte adhesion molecules-the integrins, selectins and immunoglobulin superfamily members-are reviewed with their ligands and presented in the context of leucocyte migration, activation and host defence.