Friendships and play provide children with opportunities for mutual engagement, which both require and facilitate children's language use. Modality is a semantic system in the language associated with children's learning. One way in which modality is realised is through linguistic expressions which allow speakers to moderate the degree of certainty about the truth of propositions (expressions of possibility) and the degree of necessity with respect to proposals (expressions of obligation). No research has explored the relationship between friendship and children's use of modality. This paper reports on the use of both types of modal expression during play between five- and six-year-old children in two friendship contexts. Results showed that possibility expressions occurred at similar rates in both contexts while obligation expressions occurred significantly more often in play in high-level friendships than in low-level friendships. Findings suggest that in high-level friendships modality was used cohesively to create and extend pretend play, whereas in low-level friendships it was used to regulate behaviour. Because modal expressions are important resources for managing interpersonal relationships, for talking about ideas and situations beyond the here and now and for writing for a non-present audience, our findings provide valuable insights for educators of young children.