"The Mirage" describes the widely held perception among education leaders that they already know how to help teachers improve, and that they could achieve their goal of great teaching in far more classrooms if they just applied what they knew more widely. However, research suggests that despite enormous and admirable investments of time and money, education leaders are much further from that goal than has been acknowledged, and the evidence base for what actually helps teachers improve is very thin. Like a "mirage," it is not a hallucination but a refraction of reality: Growth is possible, but the goal of widespread teaching excellence is further out of reach than it seems. This report presents findings from surveys and interviews and offers recommendations including: (1) "Redefining" what it means to help teachers improve; (2) "Reevaluating" existing professional learning supports and programs; and (3) "Reinventing" how we support effective teaching at scale. The following are appended: (1) The Mirage: Technical Appendix of Data and Analysis; (2) Detailed Summary Method for Estimating Teacher Improvement Spending; and (3) Overview of the Development Profile Analysis. Endnotes are also included.