Young children with severe communication and relationship disorders, often diagnosed with pervasive developmental disorders, respond to intensive intervention started at early ages. These disorders may be secondary to various sensory and motor processing deficits which can be conceptualized as Multisystem Developmental Disorder (MSDD). Diagnostic understanding of variations in multisystem disorders can now be linked to integrated treatment models that take into account the individual differences and developmental level of each child. Clinical vignettes describe different patterns typical of MSDD, a familybased interactive-relationship model needed by all children, and related intervention approaches to consider for each pattern. Keywords: autistic spectrum disorder, behavioral models, floor time, integrated intervention, multisystem developmental disorder, pervasive developmental disorder, relationship-based treatment