In this investigation, more than one hundred valves from forty seven fired engines have been examined. The worn surfaces of valve and insert seats from a wide variety of engines were studied using optical microscopy, SEM and EDX. The purpose of this investigation is to obtain a fundamental understanding of the primary wear mechanisms on the valve/insert seat of the engines. The wear mechanism which dominates the valve/insert seat wear depends on the engine operating conditions, seat contact configurations, valve/insert seat materials, and dynamics etc. Besides adhesion, corrosion and abrasion wear mechanisms, this study also found that shear strain is a major factor contributing to engine valve/insert seat wear. When shear strain at the seat surface exceeds the limit of the material, delamination wear is produced. Seat insert materials containing lubricants offer the potential of reducing adhesion and shear strain associated wear by reducing the coefficient of friction at the interface of the valve seat and insert. Fatigue, though it is not a wear phenomenon, is also a major factor contributing to the catastrophic valve failures. Usually, mechanical fatigue results in chordal or circumferential fractures initiated at the fillet, while thermal fatigue produces radial cracks on the valve seats initiating in the seat contact. Thermal fatigue related valve failures can be minimized by eliminating the causes of improper valve seating.