This presentation introduces the author’s accumulated long experience in applying state of the art NDT methods for post tensioned bridges’ grout injection evaluation together with minor invasive investigations for validating NDT results and for evaluation of prestressing system condition including duct’s grout injection and prestressing steel corrosion. The injection grout (cementitious based grout) has a crucial role in protecting the prestressing steel against corrosion caused by different environmental stresses. Corrosion of prestressing steel may cause dangerous fracture mechanisms such as hydrogen embrittlement and stress corrosion cracking. In addition, the injection grout has the important role of intermediating the prestressing forces from the prestressing steel to the concrete structure. Therefore, successful grout injection is critical for the correct structural functionality of a post tensioned bridge. The introduced NDT methods (ultrasound tomography and Impact-Echo), used together with invasive investigations, have already been successfully applied by the author in over 110 inspections of post tensioned bridges in the past 12 years, mostly in Finland. In situ evaluation of NDT testing results pinpoint the logical locations for invasive investigations. These include opening of tendon ducts and evaluation of the steel duct, injection grout and condition of the prestressing steel (ocular corrosion evaluation). There are thousands of aging post-tensioned bridges, frequently on critical transportation routes, built mostly in the 1960s-1970s. Occasionally the prestressing was applied in a faulty manner due to various reasons. In some cases, when stress from the environment is added to faulty construction, the consequences may be severe and possibly affect the safe usage of bridges. Faults and damages in prestressing systems are hidden from the eyes. Therefore the severity of the situation is unknown and it may result in extreme cases even in sudden collapse of bridges - as witnessed in the 2018 catastrophic collapse of the Morandi Bridge in Genoa Italy where apparently, according to publications, a post tensioned main structural part failed. Significant part of the aged post tensioned bridges have already reached renovation time. Therefore, their condition should be assessed. Bridge condition evaluation usually involves “traditional” inspection techniques which do not provide information on the internal prestressing system even though it has a crucial role in the bridge load carrying capacity. The abovementioned NDT methods combined with invasive investigations enable this critical evaluation task. In addition, those abovementioned NDT methods are suitable as quality assurance measures in newly constructed post tensioned bridges for ensuring long service life of bridges since day one.