In this paper, I argue that a constitutionalized Harm Principle could ensure that people are not jailed unless they deserve it. I do not aim to outline every possible type of bad consequence beyond harm that might be sufficiently serious to justify criminalization. Instead, I focus on criminalization that is backed up with jail terms and I argue that wrongful harm to others provides the only moral and constitutional justification for sending people to jail. Imprisonment harms the prisoner, so she (...) should not be imprisoned unless she has caused proportionate harm to others. I argue that the sufficient conditions for sending an offender to jail are: (1) that the offender's actions have (or risk) bad consequences that are sufficiently harmful to make her commensurately deserving of penal detention; and (2) that the offender culpably (that is, with a state of mind somewhere along the intentional/reckless/gross negligence continuum) chose (aimed or attempted) to bring about those bad consequences or did so with reckless indifference. The lawmaker would need to demonstrate from the ex ante perspective that proposed offenses carrying jail sentences are a proportionate and fair way of dealing with the wrongs involved. Because jail (including short sentences of a few days) involves hard treatment (seriously harmful consequences for the prisoner) harm to others would be the only bad consequence of sufficient weight to justify a jail sentence. Jailing people for wrongful behavior that has harmless consequences would be an unjust and disproportionate response. In terms of understanding imprisonment (a physical deprivation of liberty) in the United States, it is better to refer to the Eighth Amendment of the Constitution than to the Due Process Clause. The Eighth Amendment, if read morally, could be invoked to strike down laws that carry prison sentences for wrongs that do not result in harm to others. This is because harming a person by subjecting her to the hard treatment that is involved in serving a jail term would be a disproportionate response unless the wrongdoer inflicted equivalent harm on others. I argue, that the Eighth Amendment should be interpreted in a way that accords with its overall moral aim or purpose. The Amendment's overall moral aim is to ensure that the state does not inflict unjust, oppressive, or disproportional punishments on its citizens. (shrink)