SIGNIFICANCE: Immune T cells are present in adipose tissues (AT), and the stoichiometry of the different T cell subsets is altered during diet-induced obesity (DIO). T cells contribute to the early steps of AT inflammation during DIO. Recent Advances: Many factors could potentially be responsible for this altered pro-inflammatory versus anti-inflammatory T cell balance. CRITICAL ISSUES: T cells are potentially activated in AT, which vitamin D might contribute to, as will be discussed in this article. In addition, we will review the different possible contributors to T cell activation in AT, such as the CD28 and CD154 T cell costimulatory molecules in AT. FUTURE DIRECTIONS: The potential antigen presentation capacities of adipocytes should be further investigated. Moreover, the properties of these AT resident (or migrating to AT) T cells must be further assessed. Antioxid. Redox Signal. 26, 489-500.