The idea that language skills support school readiness, predicting later self-regulation and academic success, is widely accepted. Although vocabulary is often emphasized in the developmental literature, the ability to use language appropriately in the classroom, or "social communication skills," may also be critical. This paper examined longitudinal contributions of vocabulary and social communication skills, from preschool to kindergarten, to kindergarten academic achievement (reading and math) and self-regulation (executive functions and learning behaviors). Participants were 164 children (14% Latinx, 30% Black, 56% White; 57% girls) enrolled in Head Start programs. Results revealed that initial levels and growth in vocabulary and communication skills predicted better academic achievement. Social communication skills uniquely predicted self-regulation, after accounting for vocabulary. We discuss potential mechanisms for these links and recommend that strategies to build social communication skills be incorporated in preschool interventions promoting school readiness. [This paper was published in "Social Development" v29 p783-800 2020.]