Prosecuting attorneys enjoy broad discretion in decision-making that influences criminal case outcomes, and do so with little or no public scrutiny. The study examines case screening decisions, charging decisions, plea offers, sentence recommendations, and dismissals in two moderately large county prosecutors’ offices, labeled as Northern County and Southern County, and includes statistical analyses of case outcomes, responses to hypothetical cases, and responses to a survey of opinions and priorities, as well as qualitative analyses of two waves of interviews and focus groups. Researchers found that prosecutors’ decisions were guided by two basic questions, “Can I prove the case?” and “Should I prove the case?”, and the relative influence of these questions was found to shift over the course of a case. The first question was most influential at the outset of a case, where the objective strength of evidence was the determining factor in most screening decisions. Later, factors such as the seriousness of the offense, the defendant’s criminal history, characteristics of the defendant and victim, and contextual factors became increasingly influential, as prosecutors evaluated whether a case should go forward. While prosecutorial discretion is generally seen as very broad and unconstrained, prosecutors often rely on a fairly limited array of legal and quasi-legal factors to make decisions, and their decision making is further constrained by several contextual factors. These contextual constraints—rules, resources, and relationships—sometimes trump evaluations of the strength of the evidence, the seriousness of the offense, and the defendant’s criminal history. Future evaluations of prosecutorial outcomes should consider these contextual constraints when assessing the impact of case-level factors, and chief prosecutors and criminal justice policy makers should be alert to the potential for contextual factors to influence and possibly distort the exercise of prosecutorial discretion.