Legislative history is a controversial element of legal research, because some of the means and extent of its application, it is argued by some, can be irrelevant to the understanding and interpretation of the law as it is legislated in the form of statutes.
Legislative history generally consists of any materials that record the means by which legislation is enacted into statutes. Some materials can be as precise as witness testimony of a legislative session or even the written or recorded minutes of a legislative session.
Where legislative history becomes controversial in some circles is because it can lead to charge of intentionalism, which incorporates the notion of intent into the perception and understanding of law.
The fear is that by incorporating legislative history into the judicial review process will allow judges to decide precedent and opinion on political issues and political arguments presented in another venue of government, undermining the separation of power between the three branches of government (legislative, judicial, and executive) and the system of checks and balances.
There are arguments for intentionalism and legislative history, based on the belief that all statutes are at least in part, determined by historical context, and some feel, especially from the position of arguing against a standing law, that the context and intentions are extremely important tools to pointing out how laws passed in one context may become invalid in another.
In the simplest terms it provides more information than is necessary to determining the constitutional validity of the law by allowing them to incorporate political argument, which can account for imprecise language within laws.
Finally, most observers feel that it represents too much information for the court to consider as it determines and interprets laws, more than judiciaries can honestly incorporate fairly and accurately.