UK statistics have shown that a significant percentage of fatalities in fires have suffered from some kind of disability. In this context 'disability' relates to a person's physical or mental condition that impinges on their ability to react and move promptly in an emergency. Various evacuation modelling techniques are being adopted to study the movement of occupants during emergencies since the exposure of people to fires for experimental purposes is unethical. However, many evacuation models have ignored the effects of disability on escape potential and therefore tend to predict optimal evacuation times. Moreover, whilst providing some valuable insights into certain factors affecting occupant movement, current models are generally presented in isolation and fail to define a general framework for designing solutions to fire safety engineering problems. The purpose of this research programme was to develop a more general methodology for predicting evacuation times of mixed-ability populations. This was made possible through the development and use of a novel concept of evacuation peiformance index (EPI), which is the relative ease of evacuating a disabled person compared to an able-bodied person, founded on a consideration of the effects of disabilities and mobility aids on evacuation times. The author shows how this concept relates three aspects of fire safety, namely, individual characteristics of disabled occupants, the amount of assistance they require, and building design and environmental factors. She contends that the evacuation peifornzance index of a class of individuals is primarily dependent on these three categories. Experimental data to verify the above claim was collected from careflully monitored evacuation drills involving a group of disabled people. Their EPIs were determined along a pre-defined route from which their evacuation times were calculated. Comparisons between predicted times using the EPI concept and measured times from alternative empirical data were seen to be in reasonable agreement. An iterative design procedure is also suggested; one that is capable of predicting worst possible evacuation times by incorporating measures of EPI and escape route dimensions and details. The EPI concept provides fire safety engineering with a logical design philosophy, which is flexible and easily comprehensible. It endeavours to increase understanding of evacuation of disabled people, and provide a simplified mechanism for fire safety design and planning of evacuation procedures.