Explanations of signal design must meet three requirements: they must be logically coherent, they must explain the diversity in size of animal signals and they must explain the diversity in form of signals. Three selection pressures operating on animal signals are discussed: the degree of conflict or cooperation, the `efficacy' of signals and how signalling costs are paid. A distinction is made between cases where costs of signalling are paid in the production of the signal and cases where costs are paid as a consequence of giving a signal on the grounds that differences in signal design result. This is illustrated by reference to the example of warning coloration. It is concluded that general principles still elude us because of the numbers of different selection pressures that operate on the design of animal signals.