Heart failure occurs when the heart is no longer able to pump blood around the body properly. It can cause breathlessness, swollen feet and ankles, and tiredness. In about half of patients with heart failure, one measure of the heart's pumping function, called the 'ejection fraction', is normal. This type of heart failure is called heart failure with preserved ejection fraction, or HFpEF. HFpEF remains poorly understood. It is not clear why some people develop HFpEF, or what determines the severity of the condition. Treatment options may be limited. UK HFpEF is a study that aims to gain a better understanding of why people develop HFpEF, develop better tests to diagnosis it, identify and test new treatments, and follow the health of the people taking part over many years.