The No Child Left Behind Act of 2001 (NCLB) was a landmark U.S. federal education reform law signed into effect by President George W. Bush on January 8, 2002. It reauthorized the Elementary and Secondary Education Act of 1965 (ESEA), which had previously focused on aiding disadvantaged students through Title I funding. NCLB shifted the emphasis toward standards-based education reform, mandating standardized testing in reading and mathematics for grades 3-8 and once in high school, and establishing school accountability measures through Adequate Yearly Progress (AYP) targets. The law aimed to ensure that no child was left behind by closing achievement gaps, particularly for low-income, minority, and disabled students, by tying federal funding to performance outcomes. Schools failing to meet AYP faced consequences including corrective actions, restructuring, or loss of funding. The legislation passed with strong bipartisan support in late 2001, despite debates over funding levels and federal overreach. Critics argued it promoted 'teaching to the test' and unfairly penalized schools, leading to its partial replacement by the Every Student Succeeds Act (ESSA) in 2015. NCLB's legacy includes heightened focus on accountability but also ongoing controversies over its rigid metrics and implementation challenges.