In August 2015, The New Teacher Project published The Mirage, a report “confronting the hard truth” that districts make massive annual investments in teacher improvement and training (over $8 billion across the US), yet most teachers do not improve year over year. When TNTP set out to identify what actually works to foster widespread teacher improvement, they found no clear patterns of support. The report urges districts to redefine what it means to help teachers improve, reevaluate existing professional learning, and reinvent how effective teaching can be supported at scale.