Participants for the Breathing Together birth cohort are being recruited from five NHS Trusts across the UK: Chelsea & Westminster, Barts Health, Isle of Wight, Aberdeen (NHS Grampian), and Edinburgh (NHS Lothian). We aim to recruit 1000 newborn babies and follow them until they are 3 years old, observing the development of wheeze and asthma. We also want to look at the bacteria (called the microbiota) that normally live in our lungs where they interact with the airway cells and the immune system, helping to maintain a healthy environment.