This guide was created to support district leadership in preparing assistant principals for promotion to and success in the principalship. In particular, the guide is intended for use by those who regularly plan for, create, lead, and develop principal leadership training opportunities; those who have responsibilities in pipeline development and hiring or placement of school administrators; and those who supervise, evaluate, and lead succession planning for school leadership. The research base for the preparation of assistant principals for the principalship is emerging. This guide draws available evidence for: (1) forecasting principal vacancies; (2) identifying assistant principals for the principalship; and (3) implementing professional learning pathways to prepare identified assistant principals for the principalship. In addition to research, input from school district leaders informed the content and tools in this guide. Leaders responsible for principal pipelines in diverse school districts that vary in size (small, medium, and large), locale (urban, rural, and suburban), and demographics participated in an affinity group to inform the guide's design and content and then in a systematic review of a full draft. The Preparing Assistant Principals for the Principalship Affinity Group provided the opportunity for district leaders to discuss promising practices, challenges, and solutions to strengthening the principal pipeline through the identification and preparation of assistant principals. A detailed interview protocol provided the opportunity for district leaders to share feedback on guide content and implementation strategies, with particular attention to the clarity and usability of the guide.