Abstract: This entry addresses two fundamental characteristics of rules. The first concerns the degree of precision, detail, or complexity they embody: how finely are different sorts of behavior to be distinguished? A second aspect of legal commands concerns when a given level of detail is provided -- at the time of promulgation ("rules") or subsequent to individuals' actions, in the context of an adjudication ("standards"). These aspects of rules are considered from a perspective that focuses upon information costs and dissemination: different sorts of legal commands involve differing costs of formulation and application by private parties (deciding upon their own conduct) and adjudicators, and the character of laws also influences how well parties actually will understand the law and conform their conduct accordingly. The discussion encompasses related questions involving the role of precedent, the evolution of the law over time, legal uncertainly, and accuracy in adjudication. This entry also addresses the separate problem of how changes in legal rules should apply to prior behavior or pre-existing investments -- issues of retroactivity and transition.