The National Children's Study (NCS) was authorized by the Children's Health Act of 2000 and is being implemented by a dedicated Program Office in the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). The NCS is planned to be a longitudinal observational birth cohort study to evaluate the effects of chronic and intermittent exposures on child health and development in the U.S.. The NCS would be the first study to collect a broad range of environmental exposure measures for a national probability sample of about 100,000 children, followed from birth or before birth to age 21. Detailed plans for the NCS were developed by 2007 and reviewed by a National Research Council / Institute of Medicine panel. At that time, sample recruitment for the NCS Main Study was scheduled to begin in 2009 and to be completed within about 5 years. However, results from the initial seven pilot locations, which recruited sample cases in 2009-2010, indicated that the proposed household-based recruitment approach would be more costly and time consuming than planned. In response, the Program Office implemented a number of pilot tests in 2011 to evaluate alternative recruitment methods and pilot testing continues to date. At the request of Congress, The National Children's Study 2014 reviews the revised study design and proposed methodologies for the NCS Main Study. This report assesses the study's plan to determine whether it is likely to produce scientifically sound results that are generalizable to the United States population and appropriate subpopulations. The report makes recommendations about the overall study framework, sample design, timing, content and need for scientific expertise and oversight. The National Children's Study has the potential to add immeasurably to scientific knowledge about the impact of environmental exposures, broadly defined, on children\'s health and development in the United States. The recommendations of this report will help the NCS will achieve its intended objective to examine the effects of environmental influences on the health and development of American children.