When the standards movement began more than 15 years ago, most educators welcomed the prospect of having clearly articulated student learning goals. The publication of the first set of standards by the National Council of Teachers of Mathematics in 1989 was greeted with unprecedented optimism. Standards offered educators a direction for reform initiatives by providing consensus about what is important for students to learn and what skills they should acquire. In addition, standards brought much-needed focus to curriculum development efforts and provided the impetus for fashioning new forms of student assessment. The standards movement also posed new challenges for professional development leaders charged with ensuring that educators have the knowledge and skills needed to help all students reach the high levels of learning described by these newly defined standards. As they took up these challenges, those planning professional development efforts quickly discovered that the problems they faced were far more complex than they originally anticipated. If standards-based reforms are to succeed, powerful professional development is essential. Professional development leaders must recognize five essential understandings that are the building blocks for improving student achievement and producing standards-based work through professional learning: (1) standards are not new; (2) standards are a philosophy of learning; (3) ideas are more important than terminology; (4) good ideas can be poorly implemented; and (4) success hinges on what happens in the classroom.