Structured planning draws on the systems modelling approach of structured analysis and applies those ideas to project planning: the traditional elements of a project plan, the work breakdown structure and PERT diagram, are replaced by a levelled work Bow diagram and a deliverable dictionary. A technique for providing flexible summaries of the new project model, viewing,has been defined to assist with project tracking and control. This paper develops a formal definition of work flow diagrams and the technique of viewing in order to lay a theoretical foundation for structured planning, such that a complete set of operations to build and edit a levelled work flow diagram and deliverable dictionary can be specified and implemented in an automated project management tool.