Owners or operators of wind farms understandably wonder whether the performance of a wind turbine is improved enough to justify the cost of upgrading. Power output from a wind turbine is driven by wind input. The output-only difference, even if present, would not reveal whether the difference comes from upgrading the turbine or from the occurrence of a strong wind after the upgrade. Many upgrades involve installing a passive device to the existing turbine or adjusting existing turbine components physically. In the context of wind turbine upgrade, the data records collected before the upgrade form the non-treated data group, whereas those collected after the upgrade form the treated group. Feeding the post-upgrade wind and environmental data to the power curve model is analogous to running the old, unmodified turbine under the new conditions. The chapter discusses a few general issues encountered in the upgrade quantification effort.