The management and maintenance of the built heritage is one of the main concerns of the owners of concretestructures. Engineers wish to obtain quantitative information about concrete properties and theirvariability. Non-destructive testing (NDT) is very popular in this context as it quickly provides relevantinformation on the integrity and ageing of the material, but several kinds of indicators representativeof the concrete condition need to be evaluated. Combining NDT methods is currently considered asone of the most appropriate ways to improve the quality of the diagnosis of concrete structures. Thispaper describes a French project named SENSO (Strategy of non-destructive evaluation for the monitoringof concrete structures) devoted to developing a methodology for the non-destructive evaluation ofconcrete based on a multi-technique approach. The main objective of the project was to define the sensitivityof the techniques (ultrasonics, radar, capacitive and resistivity) and the variability of the evaluationfor each indicator concerned. To achieve this, a large experimental programme, involving arepresentative range of concretes and several indicators, was carried out. A database linking the measuredvalues by NDT and the indicators allowed the different measurements to be distinguished in termsof quality (linked to the variability) and in terms of relevance for the characterisation of each indicator.The added value of the combination of techniques was obtained by means of data fusion. The methodologydeveloped in laboratory conditions, involving both evaluation of the indicators and data fusion, wastested on full-scale models in which the indicators were not fully controlled.