This thesis draws upon data gathered during research undertaken with a grant from the Resuscitation Council (UK). It explores the use of Automatic External Defibrillators (AEDs) by laypeople, which is known as Public Access Defibrillation (PAD). Whilst an abundance of research has been undertaken about this phenomenon, it has predominately been conducted using quantitative methods; however the data I am using was collected using a qualitative approach. During the research, fifty-three semi-structured interviews were carried out. Most of these were with laypeople who had been trained to use AEDs, and nine involved those who delivered the training. These interviews were conducted at sites typical of those where these devices have been introduced, such as railway stations and airports. The geographical area of these locations covered the East and West Midlands, South and West Yorkshire, Lincolnshire and Essex. The aims of the research were quite broad and included exploring how to make training more realistic, how debriefing and support for those who had used an AED should be organised and how the interviewees perceived the technology inherent in the AED.This thesis reanalyses the data that was collected during that research and focuses on two themes. Firstly, some of the theories of technologies in transition are used to illustrate how AEDs were developed in a laboratory and progressed from that setting to become commonplace in public locations. The actor-network theory is adopted to argue that these technological devices exert an influence on the human actors in the networks that exist within society. Specifically, my analysis is informed by the work of Timmermans (1998; 1999; 1997) whose theories about external chest compressions I have developed and applied to AEDs. They suggest that these devices achieved universality, in part, through the influence of debates and medical protocols.One significant factor was that AEDs allowed for defibrillation to be redefined from a medical, to a first aid procedure. Eventually, using these devices was included in the protocols for first aid and this legitimised their use by laypeople. These theories are observable in the data through the interviewees" accounts of how they came to accept being trained to use AEDs. The experiences of those who had used an AED during a resuscitation attempt are provided and suggest that these are distressing and unpleasant events.Consequently, a key component of this thesis is a discussion of the necessity to provide psychological debriefing for those who have been involved in these incidents.The findings of this thesis suggest that laypeople who have used an AED usually have questions about the actions they took during the attempted resuscitation and need to address these with someone soon afterwards.