Legal systems in Great Britain and the United States were originally centered around case law, or judge-made law. The term common law refers to judge-made law that is found in judicial opinions. Judges hear cases involving particular parties, then issue decisions based on available precedent and on their own initiative in the absence of prior decisions. The notion that a common law existed that reflected the generally accepted values and practices of a society - and upon which judges drew to decide individual disputes - was behind this reliance on judge-made law.
The law in some subject areas still consists primarily of common law. In recent years, however, legislatures and administrative agencies have become much more active in the law-making process. Present-day legislatures adopt statutes affecting a broad range of activities. Some of these statutes may preempt earlier court decisions, either as a result of a deliberate action on the part of a legislature or inadvertently. For example, if the legislature disagrees with a court interpretation, the legislature can amend an existing statute - or enact a new statute - clarifying the particular issue upon which there is disagreement.
Administrative agencies, created and empowered by statute to carry out mandates, have also become extremely active in promulgating regulations that carry the force of law. Most such agencies also have the authority to issue rulings and interpretations of regulations, and to conduct hearings adjudicating disputes under their jurisdiction.
Under the balance of power inherent in our system, courts can declare statutes and regulations to be unconstitutional if they exceed constitutional authority or if they conflict with constitutional provisions. Thus the universe of potential authority for conducting research on a specific problem has broadened considerably from the days when case law comprised the bulk of legal authority. Even so, judicial opinions, whether they draw upon earlier common law precedent or apply or interpret statutes or regulations, are still a major source of law for the researcher. The complete picture can only be gained by reading the applicable statutes and regulations in conjunction with relevant cases.