Instability is one of the most common causes of failure after total knee arthroplasty. Although there are several contributing causes, surgical error and poor implant design selection contribute. For this reason, an accurate diagnosis is fundamental and is largely based on a thorough history and physical examination. In general, tibiofemoral instability can be classified into 3 different patterns: flexion instability, genu recurvatum, and extension instability. In this article, these 3 patterns are reviewed in greater depth.