Leaders develop best when they confront and learn to adapt to the unforgiving constraints that make leadership real. Leadership development has become a vast enterprise of competency models, assessments, courses, simulations, coaching, and carefully constructed learning experiences. Yet many programs remove the very conditions that make leadership difficult: incomplete information, competing demands, consequential choices, shifting relationships, limited authority, political risk, and pressure to act before the situation is fully understood. Participants may acquire useful concepts and perform well in the classroom while remaining poorly prepared for the environments in which leadership must occur. Designing Crucibles argues that knowledge and practice alone cannot produce the adaptive capability necessary for successful leaders. Instead, Nathan Bennett introduces a constraints-led approach (CLA)--translating research from sports science into a systematic methodology for leadership development. Leadership-development designers will learn how pressure, uncertainty, interdependence, incomplete control, and meaningful consequences can be calibrated to create "smaller crucibles": bounded experiences that reproduce the developmental logic of major career challenges without requiring leaders to endure their full organizational and personal costs. Connecting theories of adaptive leadership with actionable tools for constructing these exercises, this book offers a rigorous and practical framework for rethinking how leaders are developed.