This introduction presents an overview of the key concepts discussed in the subsequent chapters of the book. This book draws together and builds on work from curriculum thinkers, philosophers, and subject specialists. It explores the moral arguments for ambitious curriculum, before considering the relations of curriculum to “the knower”, in particular how insights from philosophy and cognitive science can provide a theoretical background to our work in curriculum. The book examines the nature and value of codified materials in curriculum work, and explore some examples such as knowledge maps, sequences, models of progression, and booklets, as well as the need to balance the putative permanence of such materials with the need for curriculum to be continuously discussed and developed. Curriculum theory is a necessity of whole-school curriculum leadership, since it allows leaders to see clearly the differences in the subjects, and to judge the level of thinking and development priorities within the subjects’ curriculum work.