Large areas of forestland in temperate North America, as well as in other parts of the world, aregrowing older and will soon transition into middle and then late successional stages exceeding100 yr in age. These ecosystems have been important regional carbon sinks as they recovered fromprior anthropogenic andnatural disturbance, buttheir future sink strength, or annual rate of carbonstorage, is in question. Ecosystem development theory predicts a steady decline in annual carbonstorage as forests age, but newly available, direct measurements of forest net CO2exchangechallenge that prediction. In temperate deciduous forests, where moderate severity disturbanceregimes now often prevail, there is little evidence for any marked decline in carbon storage rateduring mid-succession. Rather, an increase in physical and biological complexity under thesedisturbance regimes may drive increases in resource-use efficiency and resource availability thathelptomaintainsignificantcarbonstorage intheseforests well pastthe centurymark.Conservationof aging deciduous forests may therefore sustain the terrestrial carbon sink, whilst providing othergoods and services afforded by these biologically and structurally complex ecosystems.