This chapter focuses on the importance of defence planning in the development and execution of strategy. It first provides an overview of the interrelationships between strategy, politics, and defence planning, noting that war and its warfare should always be subject to the discipline of politics. It then examines the challenges faced by defence planners, and more specifically how they must acknowledge the uncertainties arising in all possible military behaviour in the future. It also considers a range of approaches to defence planning, along with principles that should guide every defence plan. Three principles are highlighted: the ‘good enough’ rule; it is legitimate and sensible to accommodate change in continuity and continuity in change; and all defence planning is about politics. Finally, the chapter explains how history can be helpful or not helpful to defence planners.