Offshore wind farms offer an important source of renewable energy worldwide. The noise created during their construction and operation, however, has the capacity to adversely affect the underwater environment. Consequently, a reliable, robust and accurate means of predicting and assessing the environmental effects of noise at an early stage is of key importance in providing an iterative process in the engineering towards an optimum design for the construction, which reduces environmental effects to a minimum or acceptable level while not unreasonably constraining the project or influencing its cost-benefit. It has been found that a key part of this process is to use appropriate and objective criteria for the principal effects of noise (hearing damage and avoidance), to estimate the degree of effect using these and a suitable predictive model, and to consider the biological consequences of this prediction with a view to determining if it is acceptable, and changing the engineering design if not. This paper investigates state of knowledge for assessment of underwater noise impacts on marine fauna and the ways this is used by offshore wind developers to minimise the risk to the environment.