The most significant development in criminal sentencing in recent decades has been the shift from indeterminate to determinate sentencing. Yet no study has systematically explored the factors leading to this shift. In this article, we provide the first analysis to explain why state legislatures enact reforms that significantly reduce both judges’ and parole boards’ discretion over criminal sentencing. First, we develop a political economy model that explains why legislatures acting in their own self interest may be motivated to enact these laws. Our model predicts that determinate sentencing reforms are more likely when there is tension among the political ideologies of legislatures, judges, and parole boards. Then, we empirically test the predictions of our political economy model using data from all 50 states over the period from 1960 to 2000. Our analyses confirm that political variables, such as divided government, are the primary influences on legislatures’ decisions to enact determinate sentencing reforms. These results are consistent with our model’s hypothesis: long histories of divided government produce clashes among the sentencing goals of legislatures, parole boards, and judges, and legislatures respond by enacting reforms that take power away from the judges and parole boards. Our conclusions are especially important given recent court cases and criticisms that challenge the future of determinate sentencing reforms.