Many or most stimuli in the environment are variable. Sometimes animals will categorize this variation, and we tend to underestimate the complexity of the mechanisms involved. Probably more often, response to variable stimuli is itself variable. Examples are explored in four areas, including the possibility that stimuli may occur in supernormal form: nest-site selection by birds, feeding and foraging, social behaviour, and motivation for general stimulation. Such variation occurs in both natural and artificial environments; indeed, there is no clear distinction between such environments. Variability in response has implications for environmental design and animal welfare, because it is often impossible to make definitive decisions on the values of specific features of the environment which are to be provided (for example, space allowance). Some progress in environmental design may be made by considering the whole environment and the animal's responses to all its features (for example by consumer demand theory). Further progress may be made by recognising that factors external to the animal's requirements must be taken into account. The most likely such frame of reference is the economic cost of providing different values of environmental features. Cost-benefit analysis is also unable to produce quantitative decisions on environmental design, but in a variable world it may provide the framework for relevant factors to be identified and considered in a structured way.