Synchrotron microtomography (S-µCT) has evolved to become an essential tool to study 3-D microstructural features with sizes within the µm range. The present work shows some successful applications of S-µCT to qualitative- and quantitatively analyze the 3-D microstructure of metal- and polymer-based composites. Special attention is put on the advantages compared to 2D characterisation methods as well as on the influence of 3-D features on the properties of the composites. The work is divided in: 1)Particle reinforced composites: S-µCT is used to understand the unusual behaviour of the coefficient of thermal expansion (CTE) observed during thermal cycling of a metal reinforced with 70 vol% of SiC particles. During heating, the pores present in the composite are closed, resulting in a marked decrease of the CTE at higher temperatures (> 300°C), while they reopen during cooling. Synchrotron diffraction is used to complement the analysis in order to determine the stresses between the components of the composite. 2)Short fibre reinforced composites: changes of the Si-morphology and of interconnectivity between Si and ceramic short fibres during creep exposure is investigated for a short fibre reinforced AlSi12 alloy. The results show a marked increase of the interconnectivity between Si and short fibres and coarsening of the eutectic Si. These changes can influence the creep resistance of these composites. 3)Continuous fibre reinforced composites: S-µCT using different resolutions is used to study the microstructure of carbon fibre reinforced composites. Two methods were developed, depending on the degree of resolubility of the fibres, to characterize the 3D microstructure of these composites regarding its reinforcement local distribution and the orientation of the fibres. The methods developed will allow to characterize the reinforcement distribution within continuous fibre reinforced materials and to correlate the results with quality criteria adopted in the stage of development of the composites.