What is this project about? This project aims to increase the number of people who survive after an "out of hospital cardiac arrest" by helping make sure that people who are trained in resuscitation feel able and confident enough to help. We will do this using text-messages to deliver behaviour change techniques. Why is it important? Every year in the United Kingdom, over 60,000 people have an "out of hospital cardiac arrest". A cardiac arrest is when someone's heart stops suddenly. When this happens outside a hospital e.g. at home or in a car park, this is known as an "out of hospital cardiac arrest" (OHCA). It results in death if the person is not resuscitated immediately. Currently, only one in ten people (10%) survive an out of hospital cardiac arrest in the UK. However, some areas in Europe have achieved higher rates of survival (up to a quarter, or 25% survive) which we'd like to match. What is currently being done about this? The areas in Europe that have achieved the highest rates of survival have achieved high rates of cardiopulmonary resuscitation (CPR) (giving mouth-to-mouth resuscitation and compressing the person's chest) by members of the public. If CPR can be started immediately, people are 4 times more likely to survive. In the UK, we are training as many members of the public as possible to perform CPR but this alone is not quite enough because only a small proportion of the people trained in CPR actually attempt it when they encounter someone in cardiac arrest. The reasons people give for not attempting CPR include things like lack of confidence in their CPR skills, being uncertain whether the person is actually having a cardiac arrest and being worried about doing harm. How will this study help? We can improve people's confidence by using something called "behaviour change techniques". They have been shown to be successful in helping people to stop smoking, lose weight or take more exercise, etc. and we think they can help with the behaviour of doing CPR. Delivering the behaviour change techniques using text-messages will allow us to stay in touch with people for a long time after their initial CPR training and to use videos, images etc. to make our messages appealing and more effective. We hope this approach will help people trained in CPR to remain confident, competent and ready to start CPR if required. What's involved in this study? We are a team with expertise in CPR training, resuscitation and behaviour change techniques. We will work with members of the public (some will have been trained in CPR), experts in CPR training, creative professionals (e.g graphic designers) to develop a text-message based programme for people to receive as part of their CPR training. The research will help us work out how to make the messages useful to those who receive them and to work out what should be included (e.g. simple text messages, aminations or short videos); when and how often we should contact people; how interactive the messages should be, etc. Once we have developed this programme of support, we will test it in a small number of people (20), so we can improve it, based on what they tell us. What happens after the study? We will apply for other funding, so that we can test our programme of support in a very large number of people. This will show how effective it is in helping people trained in CPR to be confident, competent and ready to start CPR if someone has an out of hospital cardiac arrest.