Background/Context: In the spring of 2015, about 135,000 New Jersey students--almost 20% of the test-eligible children--did not take the state's test. Opposition of this magnitude directly contradicted a central stipulation of the federal No Child Left Behind Act of 2001 (NCLB), which required states to test 95% of their eligible students to receive federal education funding. This article examines what happened in New Jersey in 2015 to produce such a large change in the implementation of state testing policy. Purpose/Objective/Research Question/Focus of Study: In this article, I use Richard Matland's (1995) integrative policy implementation theory to explain the circumstances that produced such a large change in the enactment of New Jersey testing policy in 2015. More specifically, I focus on two research questions: (1) What were the major national, state, and local factors that contributed to parent and student opt-out decisions in New Jersey in 2015? (2) How does integrative implementation theory help us to understand the different circumstances contributing to the opt-out movement in New Jersey in 2015? Population/Participants/Subjects: The article is based upon extensive document review and 33 interviews with New Jersey state policymakers, professional education association representatives, advocacy group leaders, school administrators, teachers, parents, and students. Intervention/Program/Practice: Since the 1990s, testing has become an increasingly important function of state education policy (Fuhrman & Elmore, 2004). High-stakes state testing policies were further expanded under the NCLB, which required states to test 95% of their eligible students to receive streams of federal education funding. The opt-out movement across the nation in 2015 was a major departure from this well-established policy. Research Design: This study employs qualitative interpretive research to examine how multiple actors at different levels of the New Jersey education system understood and interpreted the 2015 opt-out movement. Using Matland's (1995) ambiguity-conflict model of policy implementation I interpret the interactions between the policy design and local implementers' beliefs and goals. Findings/Results: The findings illuminate the shifting national, state, and local factors that contributed to district opt-out rates as well as variation across school levels and districts with different socioeconomic conditions. The combination of increased federal press on states, New Jersey's fast timelines for new standards and assessment adoption even as it ratcheted up accountability, and inconsistent state policy signals all contributed to backlash from teachers, community members, and anti-testing advocates. These factors illuminate many of the changing circumstances in New Jersey that fueled the dramatic opt-out movement in 2015. Conclusions/Recommendations: While Matland's integrative implementation model helps to explain the dynamics of the 2015 New Jersey opt-out movement, it does not account for additional contributing factors including changes over time, dynamics at different system levels, and consideration of a broader range of actors beyond policymakers and policy targets. Incorporating these factors can help make integrative implementation theories even more robust.