PROCOL. A concurrent object-oriented language with protocols delegation and constraints PROCOL is an object-oriented language with distributed delegation. It strongly supports concurrency: many objects may be active simultaneously, they execute in parallel unless engaged in communication. An object has exported operations, called Actions. Only one Action can be active at a time, however special interrupt Actions may interrupt regular Actions. Communication is performed via remote procedure call, or via a one-way synchronous message with short-time binding. In communications both client and server can be specified, either by object instance identifiers, or by type. Therefore client-server mappings may be 1-1,n-1, or 1-n, though only 1 message is transferred. PROCOL controls object access by an explicit per-object protocol. This protocol is a specification of the legality and serialization of the interaction between the object and its clients. It also provides for client type checking. The use of protocols in object communication fosters structured, safer and potentially verifiable information exchange between objects. The protocol also plays an important role as a partial interface specification. In addition it acts as a composition rule over client objects, representing relations with the client objects. PROCOL’s communication binding is dynamic (run-time); it functions therefore naturally in a distributed, incremental and dynamic object environment. PROCOL also supports constraints, without compromising information hiding. An implementation is available in the form of aC extension.