"Irreplaceables" are teachers who are so successful they are nearly impossible to replace, but who too often vanish from schools as the result of neglect and inattention. To identify and better understand the experience of these teachers, the authors started by studying 90,000 teachers across four large, geographically diverse urban school districts. They also examined student academic growth data or value-added results for approximately 20,000 of those teachers. While these measures cannot provide a complete picture of a teacher's performance or ability on their own--and shouldn't be the only measure used in real-world teacher evaluations--they are the most practical way to identify trends in a study of this scale, and research has demonstrated that they show a relationship to other performance measures, such as classroom observations. They used the data to identify teachers who performed exceptionally well (by helping students make much more academic progress than expected), and to see how their experiences and opinions about their work differed from other teachers'--particularly teachers whose performance was exceptionally poor. The neglect of Irreplaceables and tolerance for poor performance are two symptoms of an even larger problem, one that undermines the teaching profession itself: a near-total indifference to which teachers stay and which ones leave, no matter how well or poorly they perform. Schools retain their best and least-effective teachers at strikingly similar rates. Taken together, these findings reveal the extent to which teacher retention has been misunderstood and misrepresented for decades. The real teacher retention crisis is not simply the failure to retain enough teachers; it is the failure to retain the "right" teachers. A technical appendix is included. (Contains 15 figures and 91 notes.) [Additional funding for this paper was provided by the Laura and John Arnold Foundation. For "The Irreplaceables: Understanding the Real Retention Crisis in America's Urban Schools. Executive Summary," see ED533958. For related report, "5 Ways Principals Can Keep More Irreplaceable Teachers. The Irreplaceables: Understanding the Real Retention Crisis in America's Urban Schools," see ED533961.]