International large-scale assessments (ILSAs) are one of the most important tools policymakers and other educational stakeholders have to inform evidence based decision making for educational reform. Despite this, and the widespread use of ILSA data, results are sometimes misunderstood or misinterpreted. Here, we offer a brief guide to ILSAs and illuminate some of the important differences and commonalities within and across studies, limitations, and why they remain one of our most significant tools for education evaluation and reform. We focus on and compare the key studies, approaches, and structure of our own organization, the International Association for the Evaluation of Educational Achievement (IEA), with other ILSAs.