90-Day Cycles are a disciplined and structured form of inquiry designed to produce and test knowledge syntheses, prototyped processes, or products in support of improvement work. With any type of activity, organizations inevitably encounter roadblocks to improving performance and outcomes. These barriers might include intractable problems at particular junctures or complex issues affecting the entire system. In either case, 90-Day Cycles can serve as a mechanism for organizing existing knowledge or generating new ideas to address these thorny challenges. The opening section of this handbook contains a broad overview of the purpose and essential characteristics of a 90-Day Cycle. There follows a discussion of the different processes involved in conducting a 90-Day Cycle, from the point of inception (pre-cycle), through the three phases of the cycle (scan, focus, and summarize), to next steps or uses for the product(s) of the 90-Day Cycle (post-cycle). In addition to a detailed analysis of the different steps, these sections include tips and examples to facilitate their execution. The handbook concludes with a description of the roles and responsibilities of participants in the 90-Day Cycle. Templates of cycle documents are appended.