Air pollution law is the most consequential regulatory system most businesses have never read, and it works almost entirely through permits rather than prohibitions. This article explains the architecture: national ambient standards, state implementation plans that convert them into enforceable requirements, and the two permit systems that actually bind a facility. It covers why the same equipment is permitted differently depending on where it sits, what triggers new source review during a plant modification, and why "we only changed a component" is the single most litigated sentence in the field. It explains hazardous air pollutant standards, the operating permit that consolidates everything, and the monitoring and reporting obligations that generate most enforcement. It covers mobile sources and the unusual preemption structure that gives one state a special role, and closes with what an enforcement action actually looks like and how facilities respond.