The maintenance of buildings requires four stages of surveillance, or monitoring. These 4 areas are: early assessment of damage, detailed investigations, decision for repair work and verification of repair work. For each stage of monitoring, several nondestructive testing (NDT) technologies can be applied. Improvement was aimed for in 5 NDT technologies: concrete cover, dynamic behaviour, acoustic emission, actual prestress and light guide sensoring. In order to improve the accuracy and statistical interpretation of concrete cover measurement, some software and hardware were developed. The information used in the programs came from large scale testing. Concerning dynamic behaviour, a critical statement of applicability and efficiency was determined, based on large scale experiments. It was proved that acoustic emission analysis of concrete offers the possibility of detecting cracks by their crack border friction, provided the structure is under dynamic load. Large scale tests proved that acoustic emission analysis can be applied for the verification of repair work. First applications of micromagnetic techniques in civil engineering for the determination of actual prestress were tried. Due to the broad scattering of chemical content of the prestressing steel and the various mechanical treatments during the production process, the results of micromagnetic measurements must be treated carefully. For the determination of actual prestressing forces, a multiparameter analysis is needed. Monitoring systems such as light guide sensors have to stay stable and reliable during a period of time comparable to the lifetime of the structure itself. Tests were conducted to evaluate performance property fatigue.