Standards-based professional development can have a positive effect on classroom practice, particularly in terms of reform-oriented practices. It can also have a positive effect on student achievement. But the research is very mixed. Research/evaluation designs cannot always attribute effects to a specific professional development opportunity, either because there is no comparison group, or because professional development is often part of a multi-pronged systemic improvement strategy. In the summer of 2005, McREL researchers conducted and published a synthesis of the research about the influence of standards on K-12 teaching and student learning. The synthesis focused on three variables closely related to teaching and student learning: standards-based curriculum, standards-based instructional guidelines, and standards-based accountability assessments.