We developed Adhesive Dynamics as a computational tool to simulate biological contact phenomena. Built around a force balance, the method uses Monte Carlo sampling of appropriate probability distributions to simulate the stochastic formation and breakage of receptor-ligand bonds. We have simulated leukocyte rolling and firm adhesion, and developed comprehensive state diagrams that relate molecular properties to the dynamics of leukocyte adhesion. Novel insights gained from the simulations have allowed us to develop semi-analytic mathematical models to characterize rolling. Finally, we are now addressing the new frontier is to develop models that integrate signaling pathways with mechanical models of adhesion.