Has the gap in average standardized test scores between students from high- and low-income families widened, narrowed, or remained stable over the last 3 decades? The question is important both because the achievement gap is measure of how (un)equally educational opportunities are distributed in the US, and because the disparity in educational outcomes is a leading indicator of the degree of economic mobility. If the gap is widening, it suggests that children's educational experiences and opportunities in early and middle childhood -- in their homes, neighborhoods, childcare and preschool programs, and K-12 schools -- are becoming increasingly unequal, a sign that the growing economic inequality in the US has led to a parallel growth in educational inequality. A narrowing gap, however, would suggest the opposite: changes in early childhood or K-12 schooling have been equity-enhancing, even in the face of increased economic inequality among families. And because test scores and the skills they measure are valued in college admissions and the labor market, the trend in the test score gap may predict the trend in economic mobility several decades later.