A concerted effort is currently being made by industry to minimise any undesirable effects relating to windfarm development and operation. One potential effect of offshore windfarm development is the creation of underwater noise. Although the effect of underwater noise is not fully understood, it has the potential to disturb, and in the most severe cases, harm marine wildlife. Possible effects include attraction towards a noise source, avoidance, temporary hearing damage and permanent physical injury. The Crown Estate own most of the seabed up to 12 nautical miles from the British coast and lease some of this area for windfarm development. They have set up the COWRIE (Collaborative Offshore Wind Research Into the Environment) trust to conduct research into offshore windfarm development, who have highlighted the effects of underwater noise as one of several priority areas for research. This report is a review of available information on underwater noise related to windfarm development. It is one part of a larger study into the effect of windfarm related underwater noise, which includes field surveys of underwater noise from wind turbines and windfarm construction.