We present a monad-based semantics for a basic programming language for shared-state concurrency i.e. global-state in the presence of concurrency. We follow Plotkin and Power’s algebraic effect methodology, using an equational theory and a representation theorem for it. The theory we propose, CON, consists of 5 operators and 23 equations. We rely on the methodology’s modularity: 3 equations are new, while the rest are either commutativity laws or inherited from the theories of global-state and non-determinism.