Industrial wind power expanded rapidly since the earliest projects, and with this rapid expansion came understanding of wind power’s impacts on wildlife and how to measure and predict those impacts. Many of the earliest wind turbines began exceeding their operational lifespans >10 years ago, spawning plans for repowering with modern turbines. All wind turbines eventually wear out. Repowering can replace old turbines that have deteriorated to capacity factors as low as 4–12% with new wind turbines with capacity factors of 30–38%, and possibly sometimes better. At the same rated capacity, a repowered project can double and triple the energy generated from the project while reducing avian fatality rates by 60–90% when the new turbines are carefully sited. On the other hand, the grading needed for wider roads and larger pads can harm terrestrial biota, and can alter the ways that birds fly over the landscape. Larger turbines are usually mounted on taller towers, so the rotor-swept plane reaches higher into the sky and can kill species of birds and bats that were previously at lower risk. Slower cut-in speeds might increase bat fatalities, and faster cut-out speeds might increase bird fatalities. Repowering poses special problems to fatality monitoring and to estimating changes in collision rates. Differences in collision rate estimates before and after repowering can be due to climate or population cycles, changes in monitoring methods, and changes in wind turbine efficiency. Fatality monitoring could be more effective when it is (1) long-term, including when the older project was operating at peak efficiency, (2) executed experimentally, such as in a before-after, control-impact design, (3) largely consistent in methodology and otherwise adjusted for inconsistencies, and (4) sufficiently sampling the projects’ installed capacity. Another challenge is overcoming public and regulator impatience over documented wildlife fatalities. Fatality monitoring before repowering necessarily reveals project impacts. Repowering can reduce those impacts, but this message needs to be delivered effectively to a public that might be skeptical after seeing the earlier impacts and will want to see trustworthy fatality predictions going forward. Accurately predicting impacts at repowered projects can be challenging because the often-used utilization survey has fared poorly at predicting impacts, and because flight patterns can shift in the face of larger wind turbines and an altered landscape.