AbstractThe inflexibility of plans and planning has been much criticised. While there are certain material factors in the structures of developed societies which promote such inflexibility, the criticism, if substantiated, clearly represents a major failing of planning methodology. In this paper the two most prominent planning methodologies—rational comprehensive and incrementalist—are reviewed, and found to incorporate inflexibility in distinctive ways into their procedures. Other approaches to planning which stress the importance of flexibility and attempt to make it operational are then critically surveyed; from these certain desirable characteristics of a methodology for flexible planning are distilled. Finally, the urgency of the problem is illustrated with examples both from theorists of planning and from the practice of planning in a wide range of application areas. (An alternative methodology is proposed in a companion paper.)