Background: Research shows that educators can identify half of future high school dropouts as early as Grade 6--and three quarters or more of future dropouts by Grade 9--by monitoring readily available data on attendance, behavior, and course performance. These data have come to be known as the ABCs of dropout prevention, and the measures are some of the more commonly used early warning indicators. Purpose: Developing and deploying an early warning indicator system (EWIS) is a complex endeavor for school districts, especially smaller districts without access to a large internal research team or experience implementing similar data-oriented initiatives at scale. In this chapter, we describe four key considerations that school districts and others should take account of when designing an EWIS: building broad consensus on the purpose of the system, planning around system design and data availability, validating indicators and setting thresholds, and implementing and monitoring the system. Research Design: This article is based on a review of the relevant literature on designing EWISs and our experiences. We have partnered with multiple districts and states in recent years and advised these organizations on early warning indicator selection, as well as system development. References to the literature are noted where applicable. The omission of references indicates that that portion of the text is based on our experiences working directly with districts and states. Conclusions/Recommendations: Developing early warning indicators--and an EWIS--is a complex technical and human undertaking, akin to implementing a large districtwide initiative at scale. Districts of any size are well served by thinking carefully about the process and going slowly, if needed, to ensure they get the details right. Planning the system and building consensus for implementation, choosing indicators that are right for the district context, and validating and setting the right thresholds for those indicators are essential for building widespread leadership, staff, and community support for the EWIS, as well as developing the conditions for effective implementation and use.