In the American criminal justice system, prosecuting attorneys arguably enjoy broader discretion than any other system actor. Research, however, is beginning to show that prosecutorial discretion is not nearly as unconstrained as initially thought. Relying on in-depth interviews and surveys of prosecutors in two large urban/suburban county prosecutors’ offices, this article examines prosecutors’ decision making processes, exploring internal and external, formal and informal mechanisms that regulate prosecutors’ decision making. We find that prosecutorial discretion is constrained by several factors. Internal rules or policies within the prosecutor’s office often determine whether a case is accepted or rejected for prosecution, what the appropriate charge in the case should be, or what the appropriate plea offer should entail. The lack of resources in the prosecutor’s office and the local court system often require prosecutors to reject, dismiss, or amend charges in order to work within available resource limits. Relationships with law enforcement officers, judges, and defense attorneys often alter how a case will be handled. All three of these constraints – rules, resources, and relationships – can significantly influence case outcomes. Moreover, these constraints often trump evaluations of case level factors (e.g. strength of the evidence, severity of the offense, defendant criminal history), forcing prosecutors to make decisions that they may not make in the absence of such constraints.