In recent decades, the number of undocumented immigrants residing in developed countries has grown significantly. Their nebulous presence undermines states' ability to exert control over their populations and territories. In response, most host states have implemented large-scale regularizations to legalize undocumented immigrants' status. Yet we possess limited understanding of the impact these policies exert on the states themselves. This article examines the relationship between immigrant regularization, state capacity, and sovereignty. I argue that regularizations reveal a contradiction between popular images of state sovereignty and a state's actual capacity to wield authority. On one hand, regularizations facilitate the collection of information, enhancing taxation capacity, immigration control, and the rule of law. On the other, by implementing these policies, states concurrently demonstrate an inability to regulate migration and punish transgressors, thereby undermining perceptions of state sovereignty. Thus regularizations can transform states by revealing the tension between ideal images and concrete practices.