Three classifications of signals are discussed. The first is based on the type of information that they convey. A 'self-reporting signal' provides information about some property of the signaller, while an 'other-reporting signal' conveys information about an object or organism other than the signaller. The second classification is based on the processes that maintain the reliability of the signal. A 'minimal signal' is one whose cost is no greater than that needed to transmit the information effectively; such signals can only be reliable if signaller and receiver place the possible outcomes of the interaction in the same rank order. A 'cost-added signal' is a signal whose cost is greater than that required to transmit the information. The intensity of the signal is correlated with some quality of interest to the receiver. This correlation exists because, in the past, receivers have responded differently to signals of different intensity. An 'index' is a signal in which there is, again, a correlation between intensity of signal and quality, but one which does not depend on past coevolution, but on physical necessity. For example, if size is correlated with fighting ability, a signal conveying information about size is an index; in species in which the male feeds the young, courtship feeding is an index of his ability to do so. Since in both cost-added signals and indices, there is a correlation between signal intensity and some relevant quality of the signaller, both are examples of evaluation signals; this term can be used when the mechanism responsible for the correlation is uncertain. A third classification of signals is based upon the relationship between their form and that of the object being signalled about. A 'symbol' is arbitrarily linked with its object, an 'icon' resembles its object and an 'index' is physically related to its object. It seems unlikely that many, if any, biological signals are symbols.