AbstractMany interesting questions are raised by the aggressive behaviour between males of New Zealand tree wetas: (i) what is its function and does it always occur in association with galleries?; (ii) is it driven by sexual selection and is it ritualized to reduce the risk of injury?; and (iii) how do the males decide who wins? This chapter addresses these questions by reviewing the limited extant literature and by providing new data from studies of aggression behaviour in the tree wetas Hemideina femorata and H. crassidens. Seventeen behavioural units, which have been identified as components included in, but not necessary limited to, aggressive interactions, are briefly described.