Simple SummaryIntercellular adhesion molecule 1 (ICAM-1) is a protein produced by cells in blood vessel walls, as well as many other cell types. It localizes to the surface of these cells to allow them to communicate with adjacent cells. The production of ICAM-1 is induced by inflammation, such as occurs during infection, when one of its main functions is to direct white blood cells to the site of the infection. However, in autoimmune diseases and certain types of infection, ICAM-1 can be involved in directing cells to damage healthy tissue. ICAM-1 can also promote or protect against cancer, depending on the type of cancer and the cell populations that are producing it. In this review article, theICAM1gene and ICAM-1 protein are introduced, and their regulation is summarized. The roles of ICAM-1 in the immune system and a spectrum of diseases, as well as therapeutics that target ICAM-1, are discussed.AbstractIntercellular adhesion molecule 1 (ICAM-1) is a transmembrane protein in the immunoglobulin superfamily expressed on the surface of multiple cell populations and upregulated by inflammatory stimuli. It mediates cellular adhesive interactions by binding to the β2 integrins macrophage antigen 1 and leukocyte function-associated antigen 1, as well as other ligands. It has important roles in the immune system, including in leukocyte adhesion to the endothelium and transendothelial migration, and at the immunological synapse formed between lymphocytes and antigen-presenting cells. ICAM-1 has also been implicated in the pathophysiology of diverse diseases from cardiovascular diseases to autoimmune disorders, certain infections, and cancer. In this review, we summarize the current understanding of the structure and regulation of theICAM1gene and the ICAM-1 protein. We discuss the roles of ICAM-1 in the normal immune system and a selection of diseases to highlight the breadth and often double-edged nature of its functions. Finally, we discuss current therapeutics and opportunities for advancements.