Cultural heritage buildings are irreplaceable vestiges of the civilizations. The knowledge about ancient civilizations and the past of the contemporary societies is based in most cases in the study of these structures and remains. Some magnificent constructions are the cathedrals, the Christian churches that become the most representative buildings in the Middle Age European cities. These unique structures present usually a wide representation of styles and constructive techniques, being clear examples of the development and history of the societies. Unfortunately, the age and weather affect the cathedrals because all materials deteriorate due to these factors. In addition, natural or fortuitous hazards as earthquakes or fires can cause damage in their structures. As a result, restorations and conservation are usual tasks. However, maintenance of those huge and impressive monuments can only be done when the architects and engineers have enough information about the structure of the building and the possible damage. Therefore, the previous and careful structural assessment of the building becomes a decisive task for their preservation. Furthermore, in most cases, the cathedrals are built over most ancient remains. Sometimes over the most important buildings of previous civilizations, and some structures could remain hidden under the present construction. Other times, Roman or Greek temples, or Arabian mosques were partially demolished or enlarged to be transformed into cathedrals. Even older Visigothic churches were also transformed in more important structures, being hidden under it, or used partially in the construction of the new cathedral. Geophysical surveys are carried out in many cases to provide information related to these two aspects: existence of older remains under the Cathedral and definition of structural health, detection of possible damage or even, evaluating possible causes of the damage. This paper present three case study in different Cathedrals, describing three different problems and the methodologies used in the assessment of the buildings.