We propose PiCO, a calculus integrating concurrent objects and constraints, as a base for music composition tools. In contrast with calculi such as NiehrenMueller: Free, milner. parrow. ea: calculus-mobile or TyCO vasconcelos: typed-concurrent, both constraints and objects are primitive notions in PiCO. In PiCO a base object model is extended with constraints by orthogonally adding the notion of constraint system found in the ρ-calculus OzCalculus. Concurrent processes make use of a constraint store to synchronize