Studies investigating the influence of early-life pet exposures on the risk of childhood asthma have been inconsistent: some studies have observed a decreased risk whilst others have observed no effect or even an increased risk of asthma following early-life pet exposure. This study aims to use data from several longitudinal studies to investigate how exposure to pets during pregnancy or early-life influences the likelihood of: a) the immune system recognising allergens (allergic sensitisation); b) developing asthma. The study will also investigated the relative influence of type (dogs vs. cats), timing (pregnancy, early life, never) and degree (number) of pet exposures on allergic sensitisation and asthma.