Australian histories of twentieth-century criminal justice systems have been under-researched. This chapter offers a corrective, outlining the changing character and focus of twentieth-century century criminal justice institutions and pointing their on-going effects today. Major changes included a vast expansion of police power and technological capacity, and an investigative focus on individual criminal’s identities, history and rehabilitative potential. The criminal trial was transformed by procedural changes that led to an extraordinary expansion of guilty pleas, so much so that trials now form a very small part of the justice process. The focus of higher courts shifted to sentencing determinations, where imprisonment or alternatives were weighed up by judges after receiving sentencing submissions of growing complexity. Despite constant evidence against its effectiveness, the twentieth century cemented the prison as a major institution of state power. Changes meant victims of crime had less legal power and standing, although the justice system continued to rely substantially on victim reports to respond to crime.