Although much of the research on atherosclerosis has focused on the intimal accumulation of lipids and inflammatory cells, there is an increasing amount of interest in the role of the adventitia in coordinating the immune response in atherosclerosis. In this review of the contributions of the adventitia and adventitial lymphocytes to the development of atherosclerosis, we discuss recent research on the formation and structural nature of adventitial immune aggregates, potential mechanisms of crosstalk between the intima, media, and adventitia, specific contributions of B lymphocytes and T lymphocytes, and the role of the vasa vasorum and surrounding perivascular adipose tissue. Furthermore, we highlight techniques for the imaging of lymphocytes in the vasculature.