This book provides a comprehensive overview of the professional development of counselors and therapists over the career lifespan. Drawing on their own extensive experience as psychotherapists, supervisors, teachers, and researchers, as well as from their own extensive study of the topic, previously published in their 1992 book The Evolving Professional Self, the authors aim to provide an update of their work that all counselors and psychotherapists will find valuable and useful. Readers are provided with empirically based conceptual knowledge that can increase their awareness of the central issues in professional development, allowing them to monitor their own development. The authors discuss the concept of development and review the research literature on practitioner development, and then provide detailed descriptions of its six phases. Aspects of each phase addressed include the developmental tasks unique to that phase; the sources of influence and the learning process which impacts therapeutic work and a sense of development; the perception of the professional role and working style; and therapists’ measures of effectiveness and satisfaction. All of this is augmented with quotes and illustrative examples from participants in the authors’ research studies. The book includes knowledge generated from research on master therapists and from the Society for Psychotherapy Research/Collaborative Research Network. The book also considers themes of professional development; struggles faced by novice practitioners; patterns of practitioner resiliency; and ways to improve training, supervision, and practice.