Prisons are the quintessential government institution, with almost complete control over the lives of the people compelled to spend time in them. Depending on how they are run and what services they provide, they have the potential to change people's paths in life for the better or the worse, or indeed to leave people untouched. Furthermore, an enormous number of people spend time in prisons, particularly in the United States, so that the impact of imprisonment has serious consequences for society. In this article, we reflect on some of the major influences that psychology has had on prisons and imprisonment. We consider the importance of the scientist-practitioner model and the extent to which psychological evidence has permeated prison policy. We illustrate with four examples of how psychologists have contributed to understanding and influencing prisons: the Stanford Prison Experiment, the scientist-practitioner work of Hans Toch, the concepts of legitimacy and procedural justice, and the risk, needs, and responsivity principles of correctional rehabilitation. Looking to the future, we imagine how psychologically informed data science could expand its reach, and discuss ways in which prison psychologists could up our game in effectively communicating and embedding the findings of psychological science.