A functional assessment-based intervention (FABI) was designed and implemented to increase the on-task behavior of David, a second-grade student in a general education classroom. David attended an elementary school that used a comprehensive, integrated, three-tiered (CI3T) model of prevention. The school's principal nominated David for Project FUNCTION after consulting with his classroom teacher. David's teacher, Ms. Jones, stated that David demonstrated high rates of off-task behavior including leaving the assigned instructional area, disrupting peers, and using materials inappropriately during morning instructional activities. When project liaisons were first introduced to David, his guardian was navigating a medical referral process to obtain a formal attention-deficit hyperactivity disorder (ADHD) diagnosis and treatment for David. Previous interventions attempted by the school's pre-referral intervention team had been unsuccessful at increasing David's level of academic engagement. Functional assessment data were used to generate the Function Matrix and the subsequent hypothesis regarding the function of David's off-task behavior. Then, the Function-Based Intervention Decision Model was used to guide the intervention design. Ms. Jones and project liaisons collaborated to determine intervention components that would address the function of David's behavior and were feasible to implement during morning instructional activities. This article outlines the processes used to devise and implement the FABI, which include: (1) procedures to conduct the functional assessment; (2) completion of the Function Matrix and Function-Based Intervention Decision Model to determine the maintaining function of the student's behavior and the appropriate intervention model; (3) implementation of intervention components; and (4) evaluation of intervention outcomes. The practicality and ease of implementation of self-monitoring to address the dual function of David's off-task behavior are documented. (Contains 4 tables and 2 figures.)