The report in this series reviews methodologies for measuring and assessing potential changes in marine mammal behaviour, abundance or distribution arising from the construction, operation and decommissioning of offshore windfarms. The report first describes impacts from offshore windfarms on marine mammals and defines the spatial and temporal scope of investigations in order to detect impacts on marine mammals. Impacts from offshore windfarming occur at very different spatial and temporal scales: Construction work, especially pile driving, may result in short-termed but long ranged impacts, whereas the operation of windfarms is expected to result in local but long-lasting impacts. The report reviews the standard methods used in studies on marine mammals in relation to offshore windfarms. Statistical power of line transect surveys using aircrafts and ships and Static Acoustic Monitoring using T-PODs is analysed from datasets obtained in German studies.