T cells constitute a heterogeneous, hierarchically organized population, comprising several maturation/differentiation states that have different capacities for clonal expansion and self-renewal. Here, we argue that the relative probabilities of proliferation, differentiation and death — the cellular events that determine the population's structure, as well as its size — are not entirely pre-programmed or fixed; instead, these events are regulated dynamically through the recurrent interaction of lymphocytes with exogenous and endogenous antigens, antigen-presenting cells and each other.