This book introduces current theories and research on disability, and builds on the premise that disability has to be understood from the dialectical dynamics of biology, psychology, and culture over time. Based on the newest empirical research on children with disabilities, the book overcomes the limitations of the medical and social models of disability by arguing for a dialectical biopsychosocial model. The proposed model builds on Vygotsky’s cultural-historical ideas of developmental incongruence, implying that the disability emerges from the misfit between individual abilities and the cultural-historical activity settings in which the child with impairments participates.