Strong school leaders play a vital role in accelerating student achievement across a state. Research has found that principals account for 25 percent of a school's total impact on student achievement, and an above-average principal can raise student achievement by as much as 20 percentage points more than an average principal--equivalent to seven months of additional learning. Teacher surveys support these findings, with more than 90 percent saying that great leadership has a very important impact on student achievement. Recently, the question of how to raise the quality of teachers has dominated the public discourse, but states have overlooked the fact that expanding principal capacity is crucial to achieving that goal. As a result, state education agencies have devoted a lopsided proportion of their time and resources to developing and implementing new teacher evaluation systems but have invested little in the principals who are largely responsible for carrying out those evaluations. In this article, Ben Fenton illustrates how teacher quality and principal quality are deeply intertwined. In doing so, he discusses the attributes of an effective principal evaluation process, how to develop strong principal standards in one's state (the most critical of which are instructional leadership, talent management, and culture building), and the best way for states to design a strong and simple evaluation tool. The remainder of the article discusses implementing robust evaluations, bringing educators into the process, building principal supervisor capacity, and questions state boards of education should ask themselves to determine where to focus attention and resources. Fenton concludes that principals are at the front line of national reform efforts and are a key lever for ensuring their success. They lead implementation of new, more rigorous college- and career-ready standards and assessments and carry out a far more complex and in-depth teacher evaluation process than in the past. Yet policies and programs around principal quality and development have not kept up. Most states spend far more on teacher evaluations and professional development than they do on principals--a counterproductive omission. Teacher quality cannot be improved at scale without great principals.