Summary: Prisons exist mainly to protect society, not to rehabilitate inmates. Crime is subject to a power law, according to which a small minority of recidivists commit a significant proportion of crimes. People sent to prisons in the United States tend to be recidivist criminals who have been arrested an average of nine times, and more than 75% of them have had at least five arrests. Less than 5% of those sent to prisons in the United States had only one arrest on their record that led to a conviction. Criminals self-report committing dozens of crimes for every offense that results in an arrest, so official arrest figures do not reflect the true number of crimes. Because repeat offenders commit so many crimes, imprisoning a small number of criminals prevents a disproportionate number of crimes. Preventing criminals from committing crimes physically by incarcerating them reduces the number of crimes. A natural experiment conducted in Italy showed that mass releases of prisoners under collective clemency resulted in a significant increase in crime, suggesting that prisons prevented these crimes. According to estimates cited, one year of imprisonment prevents about 6-14 crimes among juveniles and 5-8 among adults. Levitt (1996) estimates that imprisoning one additional prisoner reduces crime by about 15 per year. Deterrence works mainly by increasing the likelihood of getting caught, and random policing at hot spots in Minneapolis has reduced crime by increasing police presence. Increasing the length of sentences has a weaker deterrent effect than increasing the likelihood of being caught, but longer sentences still reduce crime by preventing people from committing crimes. The article argues that Nordic-style rehabilitation is not clearly responsible for the low recidivism rate, since recidivism in the United States is not significantly higher than in Denmark/Finland/Sweden, and Norway's low recidivism rate predates the 1990s reforms. The recommended policy is to increase law enforcement and detection rates and impose significantly longer prison sentences on repeat offenders based on prior offenses, rather than treating each crime separately.