This thesis presents a normative yet practical account of how Scots and English criminal law should assess the culpability of careless persons. At present, the law in both jurisdictions distinguishes between two types of culpable, unjustified risktaking: recklessness and negligence. In everyday language, these concepts have blurred edges: persons are labelled “reckless” or “negligent” with little thought to the difference, if any, that exists between these terms. Although unproblematic in the “everyday” context, this laxity in definition is inappropriate in the criminal courtroom. Negligence is not usually a sufficient form of culpability for serious offences, whilst recklessness typically is. In the most serious crimes, recklessness thus marks the limit of criminal liability. The concept ought, therefore, to be well understood and developed. Unfortunately, courts both north and south of the border have had difficulty defining and distinguishing between recklessness and negligence. This thesis explores the resulting jurisprudential quagmires and contends that, in both jurisdictions, the absence of a visible theory of culpable carelessness accounts for the courts’ difficulties. It then looks to criminal law theory to construct a defensible account of culpable carelessness which can distinguish clearly between recklessness and negligence and explain the circumstances in which the latter ought to be criminally culpable. Finally, the thesis considers the practical implications of this theory.