A dialog system that includes a dialog manager to manage a conversation between the dialog system and a user, and to associate the conversation with a complex activity, and a plan engine to execute a plan script in connection with the complex activity, the plan script including a set of atomic dialog activities and logic to control a data and sequence flow of the atomic dialog activities, the set of atomic dialog activities being sub-activities of the complex activity, the complex activity being specified via a declarative activity specification language that connects the atomic dialog activities with a process.