Changes in respiration can have a profound effect on ecosystem C balance. This talk will present results from eddy covariance studies describing environment and management effects on ecosystem C flux from cool- and warm-season perennial grasslands. In addition, stable C isotope studies that partitioned belowground respiration between the rhizosphere and free-living soil organisms will provide estimates of whole-plant respiration in undisturbed field plots. Although respiration and photosynthesis can be closely coupled at the whole-field scale, changes in respiration rather than photosynthesis drive the net ecosystem C balance in many ecosystems and often less than 20% of assimilated C is harvested from perennial grasslands. During much of the year, respiration can be greater than photosynthetic uptake resulting in a net loss of C from the ecosystem. Ecosystem respiration tends to be less susceptible than photosynthesis to drought stress whereas both are equally responsive to N fertilization. Implications of changes in respiration rate on yield and ecosystems services such as soil C sequestration will be discussed.