This study will investigate the effectiveness of speech and language therapy in preschool children with severe developmental language disorders and special education needs. A developmental language disorder severely affects communication skills and has adverse effects on all aspects of a young childs development. There is a lack of information on the effectiveness of care and education provided to young children with severe language disorders. Results of effect studies are contradictory and in most cases aimed at older children. Furthermore there is a variety in organisation of care and education to children with developmental language disorders in countries over the world, so results of international studies cannot simply be generalized to the Dutch situation. In the proposed study the effects of intervention on growth in language skills will be investigated by following the progress of a large cohort systematically. Child factors, therapy factors and educational factors will be recorded as possible predictors of effect. Subsequently evidence based speech and language therapy according to a protocol will be compared to care as usual on schools for special education. The results of the study will lead to improvement of care and education that is offered to children with developmental language disorders.