When subjects learn to match a sample stimulus to a non-identical comparison stimulus, the stimuli may become equivalent, or substitutable for each other. Matching-to-sample procedures have generated stirnuI us equivalence with humans aged 3 years and upwards. Animals, however, have thus far failed tests of symmetry, one of the defining properties of equivalence. This human-animal difference suggests 'that language may be related to equivalence formation. In developmental studies by Beasty (1987), young children who failed equivalence tests later passed when taught to name the samplecomparison pairings during baseline matching trials. Naming, then, appears to be necessary for stimulus equivalence. Experiments in the present thesis further investigated equivalence formation in children and animals. The first two experiments yielded further evidence against equivalence in animals. Experiment 1 found no evidence of equivalence in the arbitrary matching performances of two chimpanzees involved in an ape-language training programme. In Experiment 2, pigeons failed symmetry tests despite receiving extensive symmetry exemplar training. The final series of studies examined naming and equivalence in 30 normal 4-5 year old children. In Experirrent 3, children often gave the same narre spontaneously to non-identical stimuli before matching them in equivalence tests. Experiments 4(a) - 6 systematically investigated common naming and showed it to be an extremely simple but effective way for naming to rrediate equivalence. As well as suggesting a functional definition of naming, the results indicated that the subjects' preexisting stimulus names may selectively interfere with equivalence formation by affecting the common naming relations introduced during the experiment. These results support the view that language is~a maJor determinant of human behaviour (Lowe, 1979; 1983) and they also emphasise the need for a functional analysis of language developrrent.