This thesis explores mass incarceration in the United States as an outcome of the evolution of liberal penal theory over the last two centuries. The first chapter analyzes the work of the 18th century Italian legal theorist Cesare Beccaria. Within the context of the thesis, the exploration of Beccaria’s work serves to describe many of the foundational principles and assumptions that arose out of liberal criminal theory in this period. This chapter demonstrates that in the minds of early liberal criminal theorists, such as Beccaria, the role of a justice system was not to merely punish those who break the law, but also to reform those found to have broken the law into productive members of society. The second chapter jumps ahead almost 100 years to the beginning of the International Penitentiary Commission (IPC). This chapter demonstrates that the IPC was influential in entrenching incarceration as a foundational element in the liberal penal system, which the IPC was attempting to popularize and promulgate. The final chapter follows the evolution of liberal penal theory in the United States following the Second World War. During this period, economists and neoliberal legal theorists, such as Milton Friedman, Gary Becker, and Richard Posner, dramatically altered the liberal consensus on crime and punishment. Whereas earlier liberal writers viewed the role of criminal punishment as a means of reforming prisoners into useful citizens, neoliberal criminal reformers theorized that it would be more efficient to view crime and criminal punishment as an economic problem, to be solved with the same tools that liberal economists used to examine the market economy. Instead of focusing on reforming the criminal, these theorists posited that the most effective way to decrease crime was to modify the criminal incentive structure. Overall, this thesis follows the evolution of liberal penal theory in the United States and will demonstrate that what began as a noble attempt to create a more humane and just penal system, focused on the reformation of the prisoner, became a behemoth of an institution that grew to an extraordinary level in an attempt to crackdown on crime. It will be argued that what was lost in this evolution of liberal criminal theory was the importance of social and economic context in the creation of criminal behaviour. / Graduate / 2019-09-11