The paper presents a cognitive model of the planning process, generalizing the theoretical architecture of the Hearst-II system. The model assumes that planning involves the activities of various cognitive "specialists" who suggest decisions at different levels of abstraction to incorporate into the plan. These specialists operate opportunistically, suggesting decisions whenever promising opportunities arise. The model is illustrated with a "thinking aloud" protocol and a computer simulation, contrasting it with successive refinement models. The proposed model partitions the planning space into levels of abstraction and uses a blackboard to manage decisions from different specialists. The model's descriptive power is demonstrated through an analysis of a planning protocol, showing how specialists respond to decisions on the blackboard to generate new decisions. The model's control mechanism involves a series of cycles where specialists execute actions, integrating consistent decisions into a complete plan.The paper presents a cognitive model of the planning process, generalizing the theoretical architecture of the Hearst-II system. The model assumes that planning involves the activities of various cognitive "specialists" who suggest decisions at different levels of abstraction to incorporate into the plan. These specialists operate opportunistically, suggesting decisions whenever promising opportunities arise. The model is illustrated with a "thinking aloud" protocol and a computer simulation, contrasting it with successive refinement models. The proposed model partitions the planning space into levels of abstraction and uses a blackboard to manage decisions from different specialists. The model's descriptive power is demonstrated through an analysis of a planning protocol, showing how specialists respond to decisions on the blackboard to generate new decisions. The model's control mechanism involves a series of cycles where specialists execute actions, integrating consistent decisions into a complete plan.