We highlight the need to focus on the underlying determinants and production mechanisms to fully understand the nature of facial expression of emotion and to settle the theoretical debate about the meaning of motor expression. Although emotion theorists have generally remained rather vague about the details of the process, this has been a central concern of componential appraisal theories. We describe the fundamental assumptions and predictions of this approach regarding the patterning of facial expressions for different emotions. We also review recent evidence for the assumption that specific facial muscle movements may be reliable symptoms of certain appraisal outcomes and that facial expressions unfold over time on the basis of a sequence of appraisal check results.