T cell homeostasis commonly refers to the maintenance of relatively stable T cell numbers in the peripherallymphoid organs.Among the large numbers of T cells in the periphery,T cells exhibit structural diversity,i.e.,theexpression of a diverse repertoire of T cell receptors(TCRs),and functional diversity,i.e.,the presence of T cells atnave,effector,and memory developmental stages.Although the homeostasis of T cell numbers has been extensivelystudied,investigation of the mechanisms underlying the maintenance of structural and functional diversity of Tceils is still at an early stage.The fundamental feature throughout T cell development is the interaction between theTCR and either self or foreign peptides in association with MHC molecules.In this review,we present evidenceshowing that homeostasis of T cell number and diversity is mediated through competition for limiting resources.The number of T cells is maintained through competition for limiting cytokines,whereas the diversity of T cells ismaintained by competition for self-peptide-MHC complexes.In other words,diversity of the self-peptide repertoirelimits the structural(TCR)diversity of a T cell population.We speculate that cognate low affinity self-peptides,acting as weak agonists and antagonists,regulate the homeostasis of T cell diversity whereas non-cognate or nullpeptides which are extremely abundant for any given TCR,may contribute to the homeostasis of T cell number byproviding survival signals.Moreover,self-peptides and cytokines may form specialized niches for the regulation ofT cell homeostasis.Cellular & Molecular Immunology.2005;2(1):1-10. T cell homeostasis commonly referred to the maintenance of relatively stable T cell numbers in the peripheral lymphoid organs. Among the large numbers of T cells in the periphery, T cells exhibit structural diversity, ie, the expression of a diverse repertoire of T cell receptors (TCRs) , and functional diversity, ie, the presence of T cells at nave, effector, and memory developmental stages. Although the homeostasis of T cell numbers has been extensively investigated. The investigation of the mechanisms underlying the maintenance of structural and functional diversity of Tceils is still at an early stage. The fundamental feature throughout T cell development is the interaction between theTCR and either self or foreign peptides in association with MHC molecules. in this review, we present evidence howow that that homeostasis of T cell number and diversity is mediated through competition for limiting resources. The number of T cells is maintained through competition for limiting cytokines, whereas the diversity of T ce lls ismaintained by competition for self-peptide-MHC complexes. In other words, diversity of the self-peptide repertoirelimits the structural (TCR) diversity of a T cell population. We speculate that cognate low affinity self-peptides, acting as weak agonists and antagonists, regulate the homeostasis of T cell diversity as non-cognate or nullpeptides which are extremely abundant for any given TCR, may contribute to the homeostasis of T cell number byproviding survival signals. More over, self-peptides and cytokines may form specialized niches for the regulation of T cell homeostasis. Cellular & Molecular Immunology. 2005; 2 (1): 1-10.