Challenging student behavior remains one of the biggest trials for classroom teachers. Understanding why a student performs a specific behavior is important in determining how to develop an intervention that targets the function of the behavior. This column focuses on how thinking functionally about behavior can help teachers understand why students engage in challenging behaviors. Specifically, three levels of functional thinking are discussed: (a) maintaining function, (b) deficits determination, and (c) intervention selection. Functional thinking is not meant to be a replacement for formal assessments; however, thinking about behavior in this way may help teachers to determine behavior function and develop a function-based intervention to alleviate the challenging behavior.