In the 16 years since the initial passage of "No Child Left Behind" (NCLB), the body of research around the impact of teachers and state policy experimentation on how to measure effectiveness have grown. This brief traces the evolution from the "Highly-Qualified Teacher" provisions of NCLB to the teacher evaluation systems of the 2011 "Elementary and Secondary Education Act" (ESEA) waivers to the present "Every Student Succeeds Act" (ESSA), which refrains from being prescriptive on how states choose to address this question, but does offer several opportunities for innovation and experimentation.