Contents: M.L. Rice, S.F. Warren, Preface. M.L. Rice, S.F. Warren, Introduction. Part I:Phenotypes of Language Impairments Within Clinical Diagnoses.C. Lord, S. Risi, A. Pickles, Trajectory of Language Development in Autistic Spectrum Disorders. H. Tager-Flusberg, Do Autism and Specific Language Impairment Represent Overlapping Language Disorders? J.B. Tomblin, X. Zhang, A. Weiss, H. Catts, S.E. Weismer, Dimensions of Individual Differences in Communication Skills Among Primary Grade Children. L. Abbeduto, M.M. Murphy, Language, Social Cognition, Maladaptive Behavior, and Communication in Down Syndrome and Fragile X Syndrome. A. Zukowski, Investigating Knowledge of Complex Syntax: Insights From Experimental Studies of Williams Syndrome. D.B. Bailey, Jr., J.E. Roberts, S.R. Hooper, D.D. Hatton, P.L. Mirrett, J.E. Roberts, J.M. Schaaf, Research on Fragile X Syndrome and Autism: Implications for the Study of Genes, Environments, and Developmental Language Disorders. Part II:Investigating Language Impairments Across Diagnostic Categories.C.B. Mervis, Cross-Etiology Comparisons of Cognitive and Language Development. S.F. Warren, Intervention as Experiment. M.L. Rice, Growth Models of Developmental Language Disorders. C. Phillips, Linguistics and Linking Problems. Part III:Neural, Genetic, and Behavioral Elements of Inherited Factors.R-A. Müller, Genes, Language Disorders, and Developmental Archaeology: What Role Can Neuroimaging Play? S.D. Smith, Localization and Identification of Genes Affecting Language and Learning. C.A. Morris, Genotype-Phenotype Correlations: Lessons From Williams Syndrome Research. Part IV:Research Action Steps for the Short and Long Term.P. McCardle, J. Cooper, Next Steps in the Study of Genetics and Language Disorders.