This study investigated how emotion-regulation would moderate the relationship between shyness and joint engagement and how joint engagement would mediate the relationship between shyness and language skills. Fifty-three mother-child dyads were observed in the laboratory according to the Communication Play Protocol (Adamson & Bakeman, 1999) when the toddlers were 24 and 30 months of age. Mothers completed the Temperament Behavior Assessment Questionnaire-Revised (Rothbart & Goldsmith, unpublished). Toddlers also completed the Peabody Picture Vocabulary Test-III (PPVT-III; Dunn & Dunn, 1997) and Expressive Vocabulary Test (EVT; Williams, 1997). The relationship between shyness and the percentage of time spent in non-symbol-infused coordinated joint engagement was moderated by a toddler’s ability to self-soothe. Shyer toddlers had significantly lower receptive language scores than less shy toddlers, and this relationship was partially mediated by the percentage of time toddlers spent in symbol-infused supported and coordinated joint engagement states. INDEX WORDS: Temperament, Shyness, Emotion-regulation, Language Skills, Joint Engagement