The ability to sequence information is fundamental to human performance. When subjects are asked to respond to one of several possible spatial locations of a stimulus, reaction times and error rates decrease when the target follows a sequence. In this article, we review the numerous theoretical and methodological perspectives that have been used to study sequence learning. The opportunity now exists to integrate evidence from different domains of cognitive science to begin to provide a comprehensive account of sequence learning. We suggest that subjects can learn sequences based on different information in a hierarchical representation, including either sequences of stimuli or sequences of responses. This learning can occur both with and without explicit awareness of the sequence. Multiple modes of learning exist and are subserved by different neural circuits.