The Court characterized preventive detention under the Bail Reform Act of 1984 as regulatory, not penal, for Congress intended not to punish defendants before their trials, but to respond to the serious social problem of crimes committed by those released on bail before their trials. The article examines the history of pretrial detention of dangerous defendants over the past 20 years and analyzes the legislative history and provisions of the Act as well as relevant case law construing those provisions. Recent data are presented that indicate that the Act is working effectively on the bail systems of several judicial districts. Even so, Salerno is seen as an unjustified expansion of detention based on dangerousness and its application may cause lower courts to skew the analyses they use in detention decisions. 240 footnotes.