Ecosystem service markets are increasingly used as a policy solution to environmental problems ranging from endangered species to climate change. Such markets trade in ecosystem credits created at restoration sites where conservation projects are designed and built to compensate for regulated environmental impacts. “Credit stacking” occurs when multiple, spatially overlapping credits representing different ecosystem services are sold separately to compensate for different impacts. Discussion of stacking has grown rapidly over the past three years, and it will generate increasing interest given the growing multibillion-dollar international market in carbon, habitat, and water-quality credits. Because ecosystem functions at compensation sites are interdependent and integrated, stacking may result in net environmental losses. Unless stacked compensation sites and impact sites are treated symmetrically in the accounting of environmental gains and losses, stacking may also cause environmental gains at compensation sites to be more fully accounted for than losses at impact sites. Stacking should be used with caution until science-based methods, which can account for the ecological relationships between distinct ecosystem credits present at a conservation site, are developed and deployed.