The hazards and operability, or HAZOP, study is a prime method for the identification of hazards on process plants. This is the first in a series of papers which describes progress the style of HAZOP. The work reported is embodied in a computer aid for hazard identification, or HAZOP emulator, HAZID. The HAZID code is one of a suite of codes developed as part of the STOPHAZ project. The present paper gives an overview of HAZID, with an account of HAZOP and HAZOP emulation, and of the issues underlying it. Companion papers describe the unit model system, the fluid model system and the evaluation of consequences, the evaluation and improvement of HAZID using case studies and other methods, and some development topics. Conclusions from the work are given in the final paper.