Harry, a fifth-grade student attending a public middle school that serves students from fifth to eighth grade, was described by the math and special education teachers as monotone, not caring about school or friends, and generally off task. Ms. Hart, the special education teacher, considered Harry's lack of attention, doodling, playing with his calculator, and putting his head on the desk to be of large concern and wanted immediate help for him. To address Harry's behavior, a functional assessment-based intervention (FABI) was designed, implemented, and evaluated using the systematic approach developed by Umbreit, Ferro, Liaupsin, and Lane (2007). The goal was to increase his academic engaged time (AET). This article describes the steps taken in designing and implementing the FABI for Harry: (1) conducting a functional assessment; (2) analyzing data collected via the Function Matrix to determine the maintaining function of the target behavior; and (3) using the Function-Based Intervention Decision Model to determine an appropriate intervention method. With this information, the authors developed an intervention that included antecedent adjustments, adjustments to reinforcement contingencies, and extinction components. They implemented the intervention during his inclusive math class. Overall, the full process occurred within a relatively brief, 6-week period, concluding in the final week of the academic year. (Contains 4 tables and 1 figure.)