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Wall paintings and mosaics, especially those of archaeological and art historical interest, represent a complex object of study. They are characterized by the presence of a highly valuable decorated surface and several underlying preparation layers, acting as interface to the structural support. The presence of hidden defects within this structure can be related to the ageing and deterioration of materials, inhomogeneities can be the result of past restoration interventions on the ancient artwork. This thesis project introduces a combined in-situ non-invasive approach to characterize ancient wall decorations and their underlying support in art historical and archaeological field. For this purpose, experimental in-situ and laboratory tests are aimed to evaluate the applicability, potentialities and limitations of the combination of four portable electromagnetic methods: - Multispectral imaging (MI), using a modified camera with visible, infrared and ultraviolet filters (300-1000 nm), applied to analysis of the decoration layer; - Holographic subsurface radar (HSR), with 6.4-6.8 GHz antenna), applied to analysis of the shallow subsurface layers; - Infrared thermography (IRT), applied to analysis of the subsurface structure; - High resolution ground penetrating radar (GPR), with a full polar 2 GHz antenna for the investigation of the internal structure of the wall. Operating with different frequencies and related different penetration depth and resolution, these techniques can provide complementary information regarding the surface (i.e. materials, pigments, degradation phenomena, restorations etc.) and its underlying layers (i.e. structural integrity, differences in materials, presence of humidity, detachments, cracks etc.). The in-situ tests were performed on some representative case studies in Italy and Greece, which date to different periods and are characterized by different site conditions and conservation state of the analyzed decorations. The in-situ experimental approach, proposed in the thesis, was integrated by laboratory tests, using additional non-invasive methods: Digital Holographic Speckle Pattern Interferometry and Stimulated Infrared Thermography (DHSPI-SIRT). The experimental approach has shown that the applicability of in-situ methods is strongly conditioned by the intrinsic characteristics of decorated surfaces (high value, geometry, degradation state), to which the use of non-invasive contact methods (GPR and HSR) is limited. The in-situ results are strongly influenced by logistics and acquisition conditions. The key contribution of this thesis is evaluation of advantages and limitations of the tested in-situ non-invasive investigation approach for the diagnostics of ancient wall paintings and mosaics. This experimental research has shown that portability, remote access, immediate visualization and interpretation of data are crucial in the development of general guidelines (non-invasive investigation protocol) for the diagnostics of wall paintings and mosaics. These characteristics are essential for efficient interdisciplinary collaboration between scientists, art historians, archaeologists, conservators and curators, aimed at correct monitoring and conservation planning of wall paintings and mosaics.