We present a monadic denotational semantics for a higher-order programming language with shared-state concurrency, i.e. global-state in the presence of interleaving concurrency. Central to our approach is the use of Plotkin and Power’s algebraic effect methodology: designing an equational theory that captures the intended semantics, and proving a monadic representation theorem for it. We use Abadi and Plotkin’s equational theory of resumptions that extends non-deterministic global-state with an operator for yielding to the environment. The representation is based on Brookes-style traces. Based on this representation we define a denotational semantics that is directionally adequate with respect to a standard operational semantics. We use this semantics to justify compiler transformations of interest: redundant access eliminations, each following from a mundane algebraic calculation; while structural transformations follow from reasoning over the monad’s interface.