This brief examines the impact of teaching on student achievement, necessary steps for ensuring teacher quality, and policies needed to support that effort. Research has identified the strong connection between student achievement and teacher preparation and skills. Efforts to generate consistently high teaching levels across all classrooms must begin with an articulated, widely accepted teacher preparation and support system that covers the entire spectrum of a teaching career. Research indicates that it is less harmful to students, and more cost effective, to recruit teacher candidates who meet ability and disposition requirements than to counsel out ineffective preservice or practicing teachers. Effective teacher preparation programs must include coherent, aligned coursework that is tied to practice and extensive, rigorous clinical classroom experiences. A state's credentialing process must be aligned with accepted teaching standards and designed to encourage ongoing professional development. Beginning teachers need on-the-job support. Practicing teachers must continuously update their skills and knowledge. Key policies that can contribute to improved classroom teaching include: align teaching standards to student content standards; train veteran teachers to mentor new teachers; ensure time for effective professional development; and rethink teacher compensation. Sidebars highlight how some colleges have promoted teacher development and quality. (SM)