When the language of a statute is unclear or ambiguous, attorneys will sometimes look to its legislative history to try to determine the intent or purpose of the legislature in enacting the statute. "Legislative History" can refer both to the legislative process (the steps a bill takes on its way to becoming a law) as well as the documents generated during that process.
Tracking down individual legislative history documents can be a tedious process. Fortunately, there are several sources which compile all of the documents so you don't have to.
The first step for using any of these sources is to find the Public Law Number for the law for which you need to track down the legislative history. You can find it along with the text of the law in any of the codified statute sets: the United States Code (KF 62 or online from the Web site of the U.S. House of Representatives); United States Code Annotated (KF 62 .W4 or on Westlaw); or the United States Code Service (KF 62 1972 .L38 or on LexisNexis). The Public Law Number will appear at the end of the statutory text (e.g. Pub.L 107-296) and will reference the number of the Congress (107th) and the chronological order in which the law was passed (296th law). Note that if there is more than one Public Law number listed, you will have to check the legislative history for each Public Law separately.
The most comprehensive source for compiled legislative histories CIS/Index (print KF 49 .C62, Law Reference, or online via ProQuest Congressional; Loyola user ID and password required). CIS/Index compiles legislative history documents for all Public Laws since 1970. Individual documents are identified with a CIS number, which can then be used to find the document in microfiche (in cabinets on the 4th floor). If you wish to avoid using microfiche, ProQuest Congressional allows you to search by keyword for legislative history documents (from the "Advanced Search" mode) and includes the full texts of most legislative history documents produced since 1990.
Online services such as HeinOnline's U.S. Federal Legislative History Library (Loyola user ID and password required), LexisNexis, and Westlaw also include compiled legislative histories for major legislation. HeinOnline is particularly helpful for finding the full texts of documents that comprise the histories of older statutes not covered by CIS (e.g. the Civil Rights Act of 1964), and also includes the Sources of Compiled Legislative Histories database, which can point you to sources (such as books and journal articles) that contain compiled legislative histories. Sources of Compiled Legislative Histories is also available in print (KF 42.2 1979 .J6, Law Reference).
Federal legislative histories of more recent legislation are also available at Congress.gov.
A keyword search using the Library's catalog (e.g. Sarbanes Oxley legislative history), can be used to locate compiled legislative histories available in the Library.
Committee Reports are usually the most significant portion of the legislative history, and United States Code Congressional and Administrative News (a/k/a USCCAN) (Call No. KF 48 and in Westlaw) includes selected Congressional reports for legislation enacted since 1948 in the "Legislative Histories" volumes.