We introduce an empirical framework for valuing markets in environmental offsets. Using newly-collected data on wetland conservation and offsets, we apply this framework to evaluate a set of decentralized markets in Florida, where land developers purchase offsets from a small number of long-lived producers that restore wetlands over time. We find that offsets led to substantial private gains from trade, creating about $2.2 billion of net surplus from 1995？2018 relative to a historical conservation mandate. Offset trading also led to large differences in hydrological outcomes, driven by significant differences between restored and existing wetlands in terms of area and location. A locally differentiated Pigouvian tax on offset transactions would have prevented $1.3 billion of new flood damage while preserving more than two-thirds of the private gains from trade.