Atrial fibrillation (AF) is a heart problem that causes an irregular heartbeat. It can cause the heart to beat more rapidly and reduce the heart's ability to pump blood around the body efficiently, causing heart failure. It also increases the risk of blood clots forming inside the heart. These clots may then be pumped out of the heart, through the blood vessels, to other parts of the body. This can cause strokes if the clots spread to the brain. AF is a common problem outside an Intensive Care Unit (ICU), where treatment is based on good, evidence-based guidelines designed to reduce the risk of problems like heart failure or strokes. Around 10% of patients treated in ICU develop atrial fibrillation as a complication of their underlying illness. Some patients will recover their normal heart rhythm before leaving the ICU, often with the help of some medical treatments. It is not known whether these patients will get AF again after leaving the ICU, or when this is likely to happen. It is also unknown whether patients who avoid AF whilst on the ICU may still be at high risk of developing it after they leave. This study will identify patients on the ICU who have no previous record of atrial fibrillation. These patients will be monitored whilst on the ICU to identify those who develop AF. Those patients who develop new onset AF will be monitored on the ward after leaving the ICU to see which patients have AF at this point. The heart monitoring will be repeated once the patients leave hospital, again to identify whether they have atrial fibrillation. Some studies suggest that AF during critical illness causes a long-term risk of recurrent AF and AF associated complications such as heart failure, stroke, and death. To understand how to minimise these risks in intensive care patients, we need to know how which patients who develop atrial fibrillation whilst in an ICU go on to have recurrent atrial fibrillation in hospital and in the community. These patients may benefit from interventions to reduce long term adverse events such as anticoagulation to reduce stroke risk.