This article explores different dimensions of the concept of negligence in the law. The first sections focus on the fundamental distinction between conduct negligence (unreasonable creation of a risk of harm), a conception that dominates tort law; and cognitive negligence (unreasonable failure to be aware of a risk either through inadvertence or through mistake), a conception that is much more important in criminal law. The last major section identifies five significant institutional functions served by a legal negligence standard: expressing a legal norm in the form of a standard rather than a rule; personifying fault; empowering the trier of fact to give content to the standard; creating a secondary legal norm parasitic on a primary legal norm; and distinguishing grades of fault. These functions reveal the distinctive significance of negligence, but also disclose numerous problems that the use of such a legal standard can pose. Careful analysis of these different dimensions of negligence clarifies certain misconceptions and has important implications. For example, the question whether "negligence" is an appropriate minimum standard of liability (e.g., for criminal punishment) is unanswerable until we identify the type of negligence at issue (conduct or cognitive) and its role in norm-definition (providing a general standard of liability for harm-creation or, instead, merely an interstitial standard applying only to some elements of a crime). Similarly, comparing negligenceto supposedly "more serious" forms of fault, such as recklessness, knowledge, and purpose, is treacherous and sometimes amounts to comparing apples and oranges. A better understanding of the different conceptions of negligence and of the distinctive institutional functions of a legal negligence standard can facilitate the development of more coherent, and more justifiable, fault criteria in criminal law, torts, and other legal domains.