行政法是法律中，有关国家行政权运作的国内公法总称。行政法涉及到行政机关运行职务时所适用之各项法律，在各法律部门中相较于民法、刑法、甚至于宪法，行政法的发展是较晚进的一门学科。虽然行政法并无具体的法典，但是德国法学家Otto Mayer曾说「宪法消逝，行政法长存」(Verfassungsrecht vergeht, Verwaltungsrecht besteht)，表达对行政法的重视与肯定。

Administrative law is the body of law that governs the activities of administrative agencies of government. Government agency action can include rulemaking, adjudication, or the enforcement of a specific regulatory agenda. Administrative law is considered a branch of public law. As a body of law, administrative law deals with the decision-making of administrative units of government (for example, tribunals, boards or commissions) that are part of a national regulatory scheme in such areas as police law, international trade, manufacturing, the environment, taxation, broadcasting, immigration and transport. Administrative law expanded greatly during the twentieth century, as legislative bodies worldwide created more government agencies to regulate the social, economic and political spheres of human interaction.