This document is the quadrennial report of the Mental Retardation and Developmental Disabilities (MRDD) Branch to the National Advisory Child Health and Human Development (NACHHD) Council. The MRDD Branch is a vital, evolving entity within the Center for Developmental Biology and Perinatal Medicine (CDBPM) at the National Institute of Child Health and Human Development (NICHD). The MRDD Branch provides the NICHD with a focus for research and research training aimed at understanding, preventing, and ameliorating MRDD and related conditions. The Branch's research portfolio includes studies on etiology, pathophysiology, screening, prevention, treatment, and epidemiology. The Branch research falls into five major categories: Center programs supported by the MRDD Branch; biomedical, genetic/genomic research, and chromosome abnormalities; biochemical and metabolic research; behavioral and biobehavioral research; and newborn screening and prenatal screening and diagnosis. In addition to providing a summary of the Branch's activities and accomplishments over the last four years, this report includes an enhanced discussion of possible future directions for the Branch over the next four years. The enhanced future directions represent an ongoing commitment by the NICHD leadership to seek outside input into the Institute's planning process and to keep the process transparent. Appended are: (1) MRDD Research Centers (MRDDRCs) Program; (2) Network on the Neurobiology and Genetics of Autism: Collaborative Programs of Excellence in Autism (CPEAs); (3) MRDD Branch Solicitations, Fiscal Year 2001 through Fiscal Year 2005; (4) Training Grants Currently Supported by the MRDD Branch; (5) MRDD Branch Personnel; (6) Branch-Sponsored Meetings and Conferences, Fiscal Year 2001 through Fiscal Year 2005; (7) Questions to the Expert Panel; and (8) Diseases and Syndromes: MRDD Branch Research Support for Fiscal Year 2004. (Contains 9 figures, 2 tables, and 22 footnotes.) [For the 2001 edition of this report, see ED454661.]