Policy makers at the federal level have embraced an educator effectiveness agenda, which in turn has driven many states across the country to rapidly develop and implement new and more complex teacher evaluation systems. It is increasingly clear that the success of these nascent teacher evaluation systems partly depends on the will, skill, and capacity of school principals, individuals who have historically been tasked with evaluating teachers. School principals have traditionally had, and will in most cases continue to have, primary responsibility for evaluating the 3.7 million public school teachers nationwide. While teacher evaluation innovations present several opportunities for improving instructional supervision and teacher quality, they also involve several challenges, especially on the part of principals. Time demands and cognitive challenges will be inevitable as principals learn about and implement new teacher evaluation systems. Simultaneously, other educational changes going to scale, including Common Core State Standards with aligned assessments and state school accountability systems, will compete for the attention of school leaders and teachers. Negotiating these changes to maximize the positive potential of evaluation reforms requires a commitment by states and districts to resources for training and support as well as policy coherence.