Most severe diseases cannot be prevented. The main reason is lack of information about their causes. The Norwegian Mother and Child Cohort Study (MoBa) is a population-based cohort with 100,000 children included from fetal life. The ambitious goal is to understand the causal chains behind diseases and the comprehensive data collection will allow many diseases to be addressed. The overall goal is to come up with good preventive measures. The purpose of this article is to inform about MoBa and to stimulate enhanced use of data collected during research.