Aspergillus is a mould which is found in soil, organic matter and sometimes in air conditioning units. Its size means that it can easily be breathed in and can cause respiratory disease, as well as pathology in other body systems including the skin and the eye. This podcast focuses on allergic bronchopulmonary aspergillosis, a condition which is present in 13% of adults and up to 50% of children with severe asthma. The podcast discusses when to suspect aspergillosis and the investigations which might be useful in primary care, as well as outlining an approach to the patient who comes to see you with concerns about mould in their home.