Leukocyte adhesion is of pivotal functional importance, because most leukocyte functions depend on cell-cell contact. It must be strictly controlled, both at the level of specificity and strength of interaction, and therefore several molecular systems are involved. The most important leukocyte adhesion molecules are the selectins, the leukocyte-specific beta2-integrins and the intercellular adhesion molecules. The selectins induce an initial weak contact between cells, whereas firm adhesion is achieved through integrin intercellular adhesion molecular binding. Although studies during the past twenty years have revealed several important features of leukocyte adhesion much is still poorly understood, and further work dealing with several aspects of adhesion is urgently needed. In this short essay, we review some recent developments in the field.