This book explores the development of therapists and counselors over their career lifespan in order to demonstrate their professional competence and evolution. The book is based on 160 intensive interviews with 100 practitioners. These interviews provide the data for the construction of an eight stage career model: conventional, transition to professional training, imitation of experts, conditional autonomy, exploration, integration, individuation, and integrity. The stage model data is then used to elaborate twenty broad themes of therapist/counselor development. These themes concern a number of issues such as professional identity, skill development, sources of influence, and methods of learning. The final chapter contrasts development with stagnation. The book concludes with appendices which include extensive interviews with three senior practitioners in the field. This book has been written for a wide audience including teachers of courses and supervisors in the therapy and counseling professions, those in training, and practitioners who are actively working in the field.