- Memory T cell compartments are maintained through a balance of input from precursors and turnover of existing memory cells. - Experiments tracking donor cells in host mice found the rate of input into memory compartments differs between environments. - A "two-compartment" model of fast- and slow-replenishing memory cells best explains the data from both clean and dirty environments. - The environment influences the establishment of memory during development but has less impact on steady-state maintenance once compartments are established.