Where teachers were once limited to traditional textbooks, informational texts, novels, and materials passed along by others, today the online marketplace is wide open, flush with copious materials that teachers might choose, often at little or no cost. But practically nothing is known about what these supplemental instructional materials actually look like and whether they are any good. Do they truly help educators deliver a high-quality curriculum? In this report, the authors led an analysis of supplemental materials for high school English language arts (ELA), an area where teachers are highly likely to supplement their core curriculum materials--sometimes because they do not have a core curriculum at all. They also partnered with four expert reviewers with experience in evaluating English-Language Arts (ELA) curricula and assessments to examine over three hundred of the most downloaded materials across three of the most popular supplemental websites: Teachers Pay Teachers, ReadWriteThink, and Share My Lesson. Their analysis addresses two sets of questions: (1) What types of materials are teachers downloading most frequently? What kinds of content do they include? and (2) How do experts rate the quality of these materials? What are their strengths and weaknesses, and what is the relationship (if any) between how experts view the quality of the materials and how teachers using them do? Supplemental materials are evaluated on both overall dimensions of curriculum quality (such as rigor and usability), as well as more discrete criteria that loosely reflect the key instructional. The study yields nine findings, including two strengths and seven weaknesses. [Written with Jennifer Dean. Foreword and Executive Summary by Amber M. Northern and Michael J. Petrilli. The four reviewers are: Jenni Aberli, Sarah Baughman, Bryan R. Drost, and Joey Hawkins.]