Evolving purposes for the United States educational system have driven legislative policy over the past 40 years, beginning with the Elementary and Secondary Education Act of 1965, reauthorized as the No Child Left Behind (NCLB) Act in 2002. However, researchers have demonstrated US policy intents are often unrealized in educational practice, calling for studies to elucidate how P-12 districts implement federal policy mandates. Organizational theory and historical patterns provide predictions for districts' responses, but fail to adequately explain how and why P-12 districts respond to federal policy mandates as they do, nor do they offer sufficient guidance for policymakers seeking to improve student outcomes. This article presents findings of two case studies conducted to describe and explain the response of P-12 districts to policy mandates introduced through federal legislation. Both cases are bounded by the 2001-2007 time period, and explore the districts' organizational responses to NCLB as reported by district and school administrators. The first case study, conducted in a mid-sized, Midwestern suburban district, was designed to test the conceptual framework and methodology. Using replication logic, a second case study in a large, Midwestern urban district allowed exploration of how varying contextual factors influence districts' responses to NCLB. Within- and cross-case analyses led to creation of the Compliance, Commitment, and Capacity Model (CCCM) to illustrate districts' responses to policy mandates. Loosely connected to the Concerns Based Adoption Model (CBAM) developed by Hall and Hord (2001), CCCM goes beyond CBAM's explanation of individual responses to change by describing organizational responses to mandates initiated through federal legislation. Findings suggest P-12 districts' bureaucratic responses to educational reform mandates are insufficient to produce deep changes in educational practice. In addition, accountability-based policies such as NCLB do not provide adequate guidance and support to build district capacity for change. Further research is suggested to elaborate the CCCM model as a tool for policymakers and district change agents. (Contains 2 figures, 2 tables and 2 endnotes.)