This paper is the first in a series of three papers that together are intended to serve as an introduction to the jury system as it currently operates in Australia. This paper deals with the incidence of jury trials in Australia (i.e. when are juries used ?). Topics covered include the frequency of jury trials, the extent to which juries are used as finders of fact in state criminal trials and civil trials, the constitutional guarantee of jury trial for Commonwealth offences tried on indictment, the use of juries to determine the mental health of an accused person, and the disappearance of juries from coroners courts. A number of preliminary matters are also explained by way of introduction and to provide some background and context for the discussion that follows. The second and third papers deal with the composition of juries (i.e. who is eligible for jury service and how are they selected ?), and the jury's role (i.e. what do juries do and how do they do it ?) respectively. Although published separately, the intention was that all three papers be read together.