This book is borne out of a collaborative research project (2004-8) between Math Recovery and CEM (Catholic Education Melbourne) funded by the Australian Research Council focusing on intervention in the number learning of low-attaining students in the first five years of school. This has involved developing new pedagogical tools (PTs) for assessment, instruction and intervention including a coherent, research-based framework for understanding the key aspects of students' developing number knowledge. It also offers guidance on innovative video-based assessment and an overview of principles of intervention instruction. This book will be a useful guide for all primary and elementary school classroom teachers and assistants, and specialist teachers, including experienced Mathematics Recovery instructors. It will also be of significant interest to teacher educators and researchers.