This book will help readers balance the essential scientific concepts underlying Acceptance and Commitment Therapy (ACT) with their clinical practice, reconnecting ACT with its behavioural therapeutic roots and Relational Frame Theory.Clinicians often struggle to understand the science (the "head") that should underpin their clinical practice/work (the "heart"). Without a core understanding of the scientific concepts underlying ACT, clinicians struggle to understand how to adapt ACT in practice for specific client or group situations and why. In response to that, this book is structured to help readers understand the why of each intervention and how to use that to guide the next move. Through a mix of explanations, personal examples, exercises for the therapist, short cases, and metaphors, the book provides a series of science driven concepts that teach the reader to use the ACT toolbox with skilful interventions.This manual is a must read for any ACT trainee or practitioner, helping them systematically connect techniques with the rationale for their use.