The COPSAC2010 mother-child cohort is a population-based longitudinal clinical study of 736 pregnant women and their 700 children in Copenhagen, Denmark. The main objective of this study is to reduce the burden of disease from asthma and other wheezy disorders, eczema, and allergy in young children and the long-term impact to humans and society. The COPSAC2010 study builds on the comprehensive experience and knowledge gained from the COPSAC2000 mother-child cohort, which has been monitored closely since 1998. Building the novel COPSAC2010 mother-child cohort, the same standard operating procedures, infrastructure, and skilled personnel were utilised, assuring the clinical aspects of the study goals were achieved. The families were monitored closely from week 24 of the mother's pregnancy. A major effort is the detailed clinical phenotyping and assessments of genetics, immunology, and microbiology.