In 1994, a Working Group was set up under the PAAG on Integrated Performance Assessments of Deep Repositories (IPAG). The aim of the group is to provide a forum for informed discussion on performance assessment (PA), and to examine the overall status of PA and specific issues identified by the group and the PAAG. The work will be carried out in several phases where the membership and tasks of the group are expected to change between phases. This document presents the report of a first phase of work completed in December 1996 and is expected to be of most interest to practitioners of repository PA and those with an interest in technical review of PA studies. A further phase of work is being planned emphasising the experience of peer review, and especially regulatory review, of PA studies.