The Individuals with Disabilities Education Act’s predecessor established a multilevel administrative and judicial review system for special education decisions, and ever since, the volume of special education cases in federal court has ballooned. Most present cases involve disputes over whether the school district drafted an individualized education program capable of providing a child with disabilities a “free appropriate public education.” But what evidence parties can bring to these disputes is not settled. Circuit courts are split on whether “retrospective evidence”—evidence that arises after the school district drafts the individualized education program—is admissible. This Note addresses present approaches to admitting retrospective evidence, suggests parsing different categories of retrospective evidence further, and ultimately prescribes rules for admitting certain categories. Especially in light of often intensive federal court factfinding mandated by the IDEA in special education cases, questions of what evidence influence answers of what outcomes.