Naive T cells have long been regarded as a developmentally synchronized and fairly homogeneous and quiescent cell population, the size of which depends on age, thymic output and prior infections. However, there is increasing evidence that naive T cells are heterogeneous in phenotype, function, dynamics and differentiation status. Current strategies to identify naive T cells should be adjusted to take this heterogeneity into account. Here, we provide an integrated, revised view of the naive T cell compartment and discuss its implications for healthy ageing, neonatal immunity and T cell reconstitution following haematopoietic stem cell transplantation.