Electrical trees are defects which can grow in polymeric electrical insulation under high electrical stresses. Their name results from their visual aspect which resembles natural trees. The growth of electrical trees is a precursor to long-term electrical failure; however, the basic mechanism of tree initiation and growth is not yet understood, and remains an important issue for high voltage engineers and equipment designers. In this paper the various techniques used for visualising electrical trees are reviewed and the key research questions concerning the mechanism of tree growth are considered. Optical techniques are mainly used, although electron microscopy adds useful information about the morphology of the defect. Most work has been focused on two-dimensional images. A novel approach using X-ray computed tomography is presented in this paper. The challenges and opportunities for the imaging community are considered. Detailed imaging of incipient trees and sub-micron detail of growth regions hold keys to the mechanisms of growth, but methods such as X-ray tomography need further enhancement to yield useful data. © 2012 IEEE.