The Common Core wave has swept the country. The expectation is in place for a rapid implementation of both the curriculum and the assessments. The author visited schools across the country as an external evaluator to review their transition plans, and he was struck by the great unevenness of the preparation across districts and schools. What are the best principals doing now to ensure that all stakeholders, especially teachers, are putting the new instructional system in place in their classrooms? This article presents what he observed. Just as when the state standards were introduced more than 10 years ago, five areas should command principals' personal attention. The best principals are being proactive to ensure that teachers: (1) Understand the standards; (2) Teach to the standards; (3) Assess the standards in ways that are consistent with the national assessments; (4) Monitor the progress of students in relation to key standards; and (5) Focus interventions on individual student performance on the weakest standards. Although the tasks for principals are the same now as they were 10 years ago, the challenges are significantly greater.