Discourse and grammar often complement each other, each imposing a different set of constraints on speakers' utterances. Discourse constraints are global, pertaining to text coherence, and/or to interpersonal relations. Grammatical constraints are local, pertaining to possible versus impossible structures (within specific languages). Yet, the two must meet in natural discourse. At every point during interaction speakers must simultaneously satisfy both types of constraints in order to communicate properly. It is also during conversational interaction that language change somehow takes place. This overview first explains and exemplifies how discourse constraints guide addressees in selecting specific grammatical forms at different points in the interaction (discourse 'selecting' from grammar). It then examines the relationship between discourse and grammar from a grammaticization point of view, demonstrating how a subset of discourse patterns (may) turn grammatical (grammar 'selecting' from discourse). The central theme is then that discourse depends on grammar, which in turn depends on discourse.