Facial expressions are critical for cross-species communication and emotional expression. Facial expression is increasingly used as a non-invasive method to assess quantitative measures of emotional state. Dogs (Canis familiaris) are closely related to humans, and understanding the emotions of heterospecies is of great significance for domesticated animals such as dogs. This review discusses and analyzes research progress on facial expressions in dogs and their relevance in assessing emotions, with the aim of complementing current behavioral or other measures to enhance animal welfare assessment.