There is considerable interest in space-based measurements of atmospheric CO2 for monitoring carbon emissions and for better quantifying the uptake of atmospheric CO2 by the biosphere. We now have available measurements from the Greenhouse gases Observing SATellite (GOSAT), launched in January 2009, which was the first satellite build specifically to measure atmospheric CO2. The Orbiting Carbon Observatory (OCO-2), scheduled for launch in 2013, will provide measurements that are even more precise than those from GOSAT. I will discuss the improved constraints on the terrestrial carbon fluxes that we can expect from these new measurements, and examine the limitations we face in exploiting these constraints to obtain an improved understanding of the global carbon cycle.