立法机构（亦称立法部门或立法机关）是现代社会中负责议定法律的机构，属于一种合议性团体，通常由所在地公民按人口比例组成。除了制定、修改与废除法律外，通常亦负责各式法案（如审批政府的公共预算要求）、监督政府运作（如听取施政报告与质询、召开公听会或听证会）等功能。如为国家层级的立法机构，还会负责运行宣战、媾和、批准条约、批准紧急状态、选举与罢黜政府高级官员、甚至立废国家元首等国家权力。而部分政府间国际组织也设有立法机构。立法机构通常称为「议会」或「议院」，但亦有使用不同名称。

在议会制之下，行政部门首长由立法部门产生，向立法部门负责。而在总统制之下，行政部门首长不由立法部门产生，而是分开选举。但不论在何种制度中，由于立法部门掌握了公共财政的大权，可以说是最根本的权力来源。

Legislation (or "statutory law") is law which has been promulgated (or "enacted") by a legislature or other Governing Body or the process of making it. Before an item of legislation becomes law it may be known as a bill, and may be broadly referred to as "legislation", while it remains under consideration to distinguish it from other business. Legislation can have many purposes: to regulate, to authorize, to proscribe, to provide (funds), to sanction, to grant, to declare or to restrict. It may be contrasted with a non-legislative act which is adopted by an executive or administrative body under the authority of a legislative act or for implementing a legislative act.