Electrical trees are degraded paths in polymeric insulation and are one mechanism of electrical failure of high voltage insulation systems. High-quality insulation system is needed to meet the increasing demand for electricity transmission and also to reduce energy loss and maintenance costs. Traditional polymer dielectrics filled with inorganic fillers are commonly regarded as the ideal design due to excellent physical and mechanical properties, but how tree propagation is influenced in these composites is still debatable even though there have been a lot of studies carried out in past years. The main objective of this project is to investigate how and why these filler particles influence the tree propagation. Electrical trees were grown in the HV laboratory using the needle-to-plane geometry and there were four sample groups made for comparison: unfilled, micro-filled, nano-filled (untreated) and nano-filled (surface treated). Previous studies have confirmed the application of X-ray Computed Tomography (XCT) imaging of electrical trees using phase contrast enhancement. In this thesis, XCT imaging was further developed for both unfilled and filled epoxy. A multi-stage treeing and imaging experiment was carried out in an unfilled epoxy sample to analyse the development of tree branches in three consecutive treeing steps. The impact of x-ray dose on the epoxy resin studied was also evaluated by using tensile tests and Fourier-transform infrared spectroscopy (FTIR). Synchrotron XCT was applied for the imaging work in both micro- and nano-filled materials and based on the 3D replicas, several quantified parameters were used to describe the electrical tree structures in different dielectric systems. Partial discharge (PD) measurement was also introduced into the treeing studies so as to discuss treeing characteristics. The achievements of this project include: the correlation between the tree volume and corresponding PD signals has been characterized and compared by the multi-stage test on an early tree structure. Electrical trees in microsilica-filled systems were successfully imaged and reconstructed in epoxy system with different load levels and their geometries were quantitatively compared. Interactions between tree channels and micro-sized fillers were also visualized in 3D replicas and have been used to illustrate the characteristics of the tree propagation in micro filled systems. A 'fine tree' growth model was developed based experimental results and also the transition from non-conducting to conducting tree are discussed, evidenced by PD measurement. The model is used to explain the impact of filler particles on tree growth, which is also introduced into the characterization of the tree growth in both micro- and nano-composites.