Mental health legislation in the United Kingdom recognized the need to identify potentially dangerous persons as early as 1743. Since then, only limited progress has been made towards achieving a better understanding of the concept of dangerousness, while policy reports and legislation have continued to use the concept. The law's reliance on this concept and advocacy for its wider use have reflected the law's failure to recognize the demonstrated limitations of dangerousness. This failure has forced decisionmakers to make decisions about individual liberty based on a concept that is ill-defined, misunderstood, and often inapplicable. There is a danger that pursuit of such an elusive concept gives it spurious respectability and detracts from efforts to improve on the existing system of decisionmaking. To understand dangerousness, it must be viewed as resulting from an interaction between individuals and their social and economic environment; it is not a permanent feature of an individual. An understanding of dangerousness in terms of its situational determinants may lead to policies aimed at modifying those determinants. Studying the processes whereby individuals are labelled dangerous and the factors which underlie such decisions may help experts achieve definitional clarity. Moreover, if detention of dangerous offenders is to be justified, some means of assessing both when treatment has succeeded and when it has failed is needed. About 80 references are supplied.