Summary: Adequate antigen stimulation can lead to permanent modifications of primed cells and to the generation of memory T cells that have astonishingly improved capacities to deal with antigen. The overall properties of memory T cells (increased survival, precocious and increased division capacities, and improved effector functions) can be used to identify this unique cell type. However, each immune response may lead to the generation of multiple primed types that do not necessarily possess all these characteristics. It is not known whether these different cell types are just side products of the immune reaction or whether they are involved in disease control. Control of different infections may involve different challenges and lead to the generation of different types of immune reactions. Our major challenge is to unravel this complexity, but we must overcome our handicapped experimental tests and our imperfect a priori definitions.