School districts are confronting difficult choices in the aftermath of the financial crisis. Today, the financial imbalance in many school districts is so large that there may be few alternatives to teacher layoffs. In nearly all school districts, layoffs are currently determined by some version of teacher seniority. Yet, alternative approaches to personnel reductions may substantially reduce the harm to students from staff reductions relative to layoffs based on seniority. As a result, many school district leaders and other policy makers are raising important questions about whether~other criteria, such as measures of teacher effectiveness, should inform layoffs. This policy brief, a quick look at some aspects of the debate, illustrates the differences in New York City public schools that would result if layoffs were determined by seniority in comparison to a measure of teacher effectiveness.