Abstract: Millions of times every year, American prosecutors make the all-important decision whether to decline or file formal criminal charges after police have made an arrest. This declination decision determines whether an arrest will become a full-fledged criminal case and thus whether an individual arrestee will become a defendant. It establishes the classic dividing line between investigation and adjudication, triggering numerous constitutional consequences. Through declination, prosecutors also check and regulate police decision-making within the executive branch. In an era of racialized mass incarceration, prosecutorial declination can function as a mode of equitable gatekeeping, regulating the impact of sloppy or biased policing practices on communities, courts, and the rest of the criminal pipeline. It is therefore a unique structural moment of institutional and constitutional significance. Declination is especially influential because police and prosecutors are the two main decision-makers within the carceral executive branch. This Article conceptualizes the relationship between them as an overlooked example of internal separation of powers, with the declination decision as its most impactful regulatory moment. Administrative law teaches that intrabranch checks are vital, especially when interbranch separation of powers has proven ineffective as it famously has with respect to the penal executive. The prosecutorial declination decision, in turn, is an especially promising intrabranch checking tool. It offers decisional friction, oversight, and accountability within the executive at precisely the moment when good law enforcement decision-making makes a big difference for millions of people. In our massive misdemeanor system, this regulatory promise usually fails. Misdemeanor prosecutors routinely rubber-stamp police arrest decisions and convert arrests automatically into formal charges: namely, they abdicate their screening and checking functions by deferring to police. Misdemeanor declination rates are typically very low—often less than five percent—which means that police effectively get to decide not only who will be arrested but who will be formally charged with a crime. This is not how the criminal system is supposed to work. In administrative law terms, such prosecutorial abdication is a violation of basic branch design and a worrisome species of intrabranch collusion. It is, however, neither universal nor foreordained. Around the country, many newly elected prosecutors have embraced strong misdemeanor declination policies, not only as a way of checking police but increasing equity, efficiency, and accountability. Such policies exemplify how misdemeanor declination is an underappreciated opportunity to regulate the penal executive from within and to mitigate the excesses and injustices of the low-level carceral state.