Investigations into the possession of online child sexual abuse images reveal enormous variety in the types of images collected by adults with a sexual interest in children, illustrating that the concept is broad, changeable and, at the margins, elusive. The use of criminal law to regulate any activity requires that the proscribed conduct be clearly defined. Tony Krone reviews the ways in which child sexual abuse images can be categorised and examines the impact of the viewer's perception on the definition of child pornography offences in Australia.