This paper discusses the importance of teacher professional development to student achievement. Section 1, "The Imperative: Investment in Human Skill and Knowledge," explains that despite contradictory evidence, prevailing assumptions asserts that teachers learn most of what they need during preservice education. Section 2, "The Knowledge Gap in Professional Development: The Ideal and the Real," examines the consensus on effective professional development. Section 3, "The Practice of Professional Development: The Real," discusses the knowledge gap and the costs of failure. Section 4, "The Practice of Improvement: Getting from Here to There," explains that professional development is a collective good, and its value is judged by what it contributes to the individual's capacity to improve the quality of education overall. Section 5, "How Professional Development Can Work to Improve Schools," discusses knowledge and skills (e.g., expertise in teaching exists, and context matters) and incentives (e.g., internal accountability precedes external accountability, and the most direct incentives are embedded within work itself). Section 6, "Effective Use of Professional Development Requires High Organizational Capacity," explains that to use professional development as an instrument of instructional improvement, schools and school systems must reorganize themselves to make substantial changes in the conditions of work for teachers and students. Section 7 offers "Conclusion: Developing the Practice of Improvement." Appended are brief summaries of standards for staff development, principles for transforming professional development, and research-based principles for improving professional development. (Contains 67 references.) (SM)