A State Implementation Plan (SIP) is a collection of regulations and documents that a state develops and submits to EPA describing how it will implement, maintain, and enforce federal air quality standards under the Clean Air Act. Each state must submit a SIP showing it will achieve and maintain National Ambient Air Quality Standards (NAAQS). SIPs include emission limits for pollution sources, monitoring requirements, and enforcement mechanisms. If EPA finds a SIP inadequate, it can reject the plan and impose a Federal Implementation Plan instead. The SIP process is central to the cooperative federalism model of the Clean Air Act, where states have primary responsibility for air quality but must meet federal minimum standards.