Dendritic abnormalities are the most consistent anatomical correlates of mental retardation (MR). Earliest descriptions included dendritic spine dysgenesis, which was first associated with unclassified MR, but can also be found in genetic syndromes associated with MR. Genetic disorders with well-defined dendritic anomalies involving branches and/or spines include Down, Rett and fragile-X syndromes. Cytoarchitectonic analyses also suggest dendritic pathology in Williams and Rubinstein-Taybi syndromes. Dendritic abnormalities appear to have syndrome-specific pathogenesis and evolution, which correlate to some extent with their cognitive profile. The significance of dendritic pathology in synaptic circuitry and the role of animal models in the study of MR-associated dendritic abnormalities are also discussed. Finally, a model of genotype to neurologic phenotype pathway in MR, centered in dendritic abnormalities, is postulated.