Human subjective and behavioural response to sound is related to physical sound levels, but can also be considerably affected by many other variables according to individual sensitivities, attitudes and opinions. This can be a problem for standards and regulations which are intended to reduce annoyance, sleep disturbance or effects on health, but which have to be based on physical quantities. It is not practicable to use actual annoyance, sleep disturbance or effects on health when setting standards and regulations or defining contracts. It is desirable that the physical metrics used for noise limits, targets and benchmarks should reflect human response, even if they are not directly connected. Some physical metrics are better than others in this respect, and this chapter sets out to explain why this is so and to outline where the major sources of uncertainty can be found. Human response to vibration is dealt with separately in Chapter 7.