The sections in this article are Physical Description Tree Generation Observable Features Accompanying Tree Formation Discharges and Tree Shape Discharge Patterns for Diagnosis Theoretical Models of Tree Formation Future Developments and Unresolved Questions

Electrical trees are branched degradation structures caused by field enhancing defects in insulating materials and are a common precursor to their failure. In this article we describe their shape and physical nature, and discuss the mechanism whereby they are generated in the insulating material together with a number of theoretical models for their formation. The influence of a range of physical factors upon their initiation and propagation is outlined and the use of partial discharges in their diagnosis briefly described. The article ends with a statement of unresolved questions and recent work in this area which is focussed on the emerging nano‐composite materials.