Repeated presentation of a single stimulus produces habituation—engages a learning process that results in a reduction of the ability of the stimulus to evoke its customary response. Repeated stimulus presentation is a feature of the standard procedure for classical conditioning, although, in this case, subjects experience repeated presentations of 2 stimuli occurring in sequence: S1–S2. We ask how habituation to each of these stimuli (S1 and S2) is influenced by this form of sequential presentation and what implications any effects might have for the understanding of both conditioning and habituation itself. Our review of the experimental evidence demonstrates no clear effect on habituation to S2 of preceding this stimulus with S1. Habituation to S1, however, is attenuated or prevented by the occurrence of S2: Some orienting responses are maintained when S2 follows S1 inconsistently; other responses (habituation of which may be taken to indicate a reduction in the effective salience of the stimulus) are maintained when a salient S2 reliably follows S1. We discuss the implications of these changes in the properties of S1 for associative theories of conditioning and, in particular, for the proposal that the rules that govern changes in the associability of a stimulus differ from those governing changes in its effective salience.