 The Japan Environment and Children's Study, JCS, is a nationwide birth cohort study that investigates environmental factors affecting children's health and development. It recruited 100,778 pregnancies resulting in delivery, with 51% involving program participation by male partners. The study included 95,248 unique mothers and 49,189 unique fathers, resulting in a coverage of children in 2013 of approximately 45%. The data on the characteristics of the mothers and children studied showed marked similarity to those obtained from Japan's 2013 vital statistics survey. This article was authored by Takahiro Michikawa, Hiroshi Nitta, Shoji F. Nakayama, and others.