Risk assessment – measuring an individual’s potential for offending – has long been an important aspect of most legal systems, in a wide variety of contexts. In most countries, sentences are often heavily influenced by concerns about preventing reoffending. Correctional officials and parole boards routinely rely on risk assessments. Post-sentence commitment of “dangerous” offenders (particularly common in connection with sex offenders) is based almost entirely on determinations of risk, as is involuntary hospital commitment of people found not guilty by reason of insanity and of people who are not prosecuted but require treatment. Detention prior to trial is frequently authorized not only upon a finding that a suspect will otherwise flee the jurisdiction, but also when the individual is thought to pose a risk to society if left at large. And police on the streets have always been on the look-out for suspicious individuals who might be up to no good.