Leucocytes travel around the body in the blood vessels but, with the exception of platelets, must normally leave the bloodstream to exert their protective influence in the neighbouring tissues. To facilitate this, complex sets of interactions between the endothelium and different categories of leucocyte have evolved. Three of these sets of interactions are described in this overview: first, the steps involved in neutrophi| binding to, and extravasation through, endothelium during an inflammatory reaction; second, the mechanisms of attachment and activation of blood platelets on interaction with damaged blood vessels and third, the elaborate series of interactions between lymphocyte subpopulations and different types of endothelium in the control of lymphocyte recirculation patterns. In each of these areas, enormous progress has been made in recent years, such that the study of leucocyteendothelial cell interactions is an outstanding, arguably the best, example of the rapid application of developments in molecular and cell biology to an important aspect of immunophysiology.