The Clean Air Act (CAA) is a piece of US federal legislation that attempts to comprehensively regulate air emissions released by area, stationary, and mobile sources. The original Act also called for the EPA to establish National Ambient Air Quality Standards (NAAQS) to protect public health and the environment.

The goal of the original Clean Air Act of 1970 was to create and meet NAAQS in every state in the union by 1975. The Act was amended in 1977 when this ambitious deadline was far from being met in most of the states. The Act was again amended in 1990 to account for the formerly unregulated issues of acid rain, ground-level ozone, stratospheric ozone depletion, and various air toxics.

The Act and its amendments are divided into several subchapters, also called titles, that cover various aspects of air pollution.

Subchapter   I   - Programs and Activities
Subchapter  II   - Emission Standards for Mobile Sources
Subchapter III   - General Provisions
Subchapter  IV   - Noise Pollution Provisions
Subchapter  IV-A - Acid Rain Provisions
Subchapter   V   - Permiting Requirements
Subchapter  VI   - Stratospheric Ozone Protection