Abstract: Few decisions in the criminal justice process are as consequential as the determination of bail. Indeed, recent empirical research finds that pre-trial detention imposes substantial long-term costs on defendants and society. Defendants who are detained before trial are more likely to plead guilty, less likely to be employed, and less likely to access social safety net programs for several years after arrest. Spurred in part by these concerns, critics of the bail system have urged numerous jurisdictions to adopt bail reforms, which have led to growing momentum for a large-scale transformation of the bail system. Yet supporters of the current system counter that pre-trial detention reduces flight and pre-trial crime—recognized benefits to society—by incapacitating defendants. Despite empirical evidence in support of both positions, however, advocates and critics of the current bail system have generally ignored the real trade-offs associated with detention. This Article provides a broad conceptual framework for how policymakers can design a better bail system by weighing both the costs and benefits of pre-trial detention—trade-offs that are historically grounded in law, but often disregarded in practice. I begin by presenting a simple taxonomy of the major categories of costs and benefits that stem from pre-trial detention. Building from this taxonomy, I conduct a partial cost-benefit analysis that incorporates the existing evidence, finding that the current state of pre-trial detention is generating large social losses. Next, I formally present a framework that accounts for heterogeneity in both costs and benefits across defendants, illustrating that detention on the basis of “risk” alone can lead to socially suboptimal outcomes. In the next part of the Article, I present new empirical evidence showing that a cost-benefit framework has the potential to improve accuracy and equity in bail decision-making, where currently bail judges are left to their own heuristics and biases. Using data on criminal defendants and bail judges in two urban jurisdictions, and exploiting variation from the random assignment of cases to judges, I find significant judge differences in pre-trial release rates, the assignment of money bail, and racial gaps in release rates. While there are any number of reasons why judges within the same jurisdiction may vary in their bail decisions, these results indicate that judges may not be all setting bail at the socially optimal level. The conceptual framework developed in this Article also sheds light on the ability of recent bail reforms to increase social welfare. While the empirical evidence is scant, electronic monitoring holds promise as a welfare-enhancing alternative to pre-trial detention. In contrast, application of the conceptual framework cautions against the expanding use of risk-assessment instruments. These instruments, by recommending the detention of high-risk defendants, overlook the possibility that these high-risk defendants may also be “high-harm” such that they are most adversely affected by a stay in jail. Instead, I recommend that jurisdictions develop “net benefit” assessment instruments by predicting both risk and harm for each defendant in order to move closer toward a bail system that maximizes social welfare.