It is often argued that an immunization strategy violates arbitrage-free equilibrium. Because immunization is a static concept, we contend that this argument is not valid. This paper examines the immunization strategy in a dynamic setting, and shows that global immunization is feasible for any arbitrage-free affine term structure model, including the parallel shift model. Further, we show that immunization does not violate arbitrage-free pricing because the cost of immunization over time is positive. Consequently, immunization strategies based upon commonly used duration measures are not theoretically unsound.