Monitoring and mitigation reports from 19 UK and 9 other European Union (EU) offshore wind farm (OWF) developments were reviewed, providing a synthesis of the evidence associated with the observed environmental impact on marine mammals. UK licensing conditions were largely concerned with mitigation measures reducing the risk of physical and auditory injury from pile driving. At the other EU sites, impact monitoring was conducted along with mitigation measures. Noise-mitigation measures were developed and tested in UK and German waters in German government-financed projects. We highlight some of the review’s findings and lessons learned with regard to noise impact on marine mammals.