This document describes the procedures and results of the air quality modeling analyses used to quantify the impacts and benefits of the Clear Skies Act (CSA) of 2003. The air quality modeling was conducted to support several components of the legislation including: an assessment of the costs and benefits associated with the Act, and an assessment of the expected impact of the Act on ozone and particulate matter levels. The air quality model applications include episodic regional scale ozone modeling for the Eastern U.S. and annual particulate matter (PM), visibility, and deposition modeling on a continental scale covering the 48 contiguous States. 1996 Base Year simulations were made to examine the ability of the modeling systems to replicate observed concentrations of these pollutants. This was followed by simulations using a 2001 proxy emissions inventory. The 2001 inventory was created for the purpose of aligning the current year ozone and particulate matter design values (1999-2001) to the emissions inventory year.