High endothelial venules (HEV) are vessels characteristic of lymph nodes specialized for promoting lymphocyte migration between blood and the nodal parenchyma. The studies outlined in this article constitute an overview of work done in my laboratory to investigate some functional aspects of HEV and a group of venules that develop in sites in lymphocyte accumulation in non-lymphoid tissues which resemble HEV in many ways and are therefore known as HEV-like vessels. This is followed by a hypothesis to explain the development of HEV-like vessels and its importance for lymphocyte traffic.