Part I discusses the negative consequences of related multiforum litigation and briefly outlines existing formal mechanisms for aggregating cases concurrently pending in both State and Federal courts. Part II discusses several proposed new mechanisms for facilitating aggregation. Part III describes how judges engaged in informal intersystem coordination, including the various ways they coordinated their litigation, the techniques they developed, the procedures that proved most successful, and how difficulties encountered in the process were or were not resolved. Part IV addresses various issues relevant to State-Federal coordination, including the factors that make a particular litigation suitable for coordination; the role of attorneys in facilitating coordination; and a variety of federalism concerns, such as deference by State and Federal judges to one another on issues of State and Federal law, and the tension between consistency and correctness. The article concludes that when litigation spans State and Federal courts, informal coordination can advance judicial economy, efficiency, and fairness. 270 footnotes