This book is grounded in the distinction between teacher development that happens and teacher development that matters. Teacher development that makes a difference is not defined by the number of workshops attended, policies implemented, or competencies ticked off. Rather, it is defined by the extent to which professional learning shapes what teachers think, feel, value, and are able to enact over time. Fundamentally, effective teacher development operates from teachers’ internal perspective. It builds psychological resources within teachers—such as professional identity, motivation, agency, resilience, ethical commitment, and reflective capacity—that enable them to engage meaningfully with the complex demands of teaching.