For more than 30 years, the Physics Education Group at the University of Washington has been engaged in a coordinated program of research, curriculum development, and instruction to improve student learning in physics (K-20).  The work of the group is guided by ongoing discipline-based research.  This type of research differs from traditional education research in that the emphasis is not on educational theory or methodology in the general sense but rather on student understanding of science content.  For both intellectual and practical reasons, discipline-based education research must be conducted by science faculty within science departments.