EPA conducted its 2005 annual review of the existing effluent guidelines it has promulgated for 56 categories of industrial dischargers. The first component of EPA's annual review was a screening-level analysis of all promulgated effluent guidelines. During the screening-level analysis, EPA estimated the pounds of toxic pollutants discharged by industrial categories with existing effluent guidelines. The second component of EPA's annual review is a more extensive review of certain categories prioritized based on their high estimated toxic discharges. EPA is conducting detailed studies of the two categories with the highest estimated toxic discharges, as well as a less detailed preliminary review for 11 additional categories. This report describes the results of EPA's 2005 preliminary review of prioritized categories of industrial dischargers. This section presents background information about the 2005 preliminary category review, in the following subsections: A. Selection of Categories for Review; B. Purpose and Scope of Preliminary Category Review; C. Data Sources; D. Category Review During 2003 and 2004 Annual Reviews; E. Structure of this Report; and F. References.