The concept of "specific developmental disorder" (SDD) refers to delays in developmental domains such as language and speech development, motor coordination or the development of scholastic skills, in the absence of sensory deficits, subnormal intelligence or poor educational conditions. The key element in this concept is the notion of a discrepancy between observed and expected level of development. In DSM-III-R and ICD-10, SDD serves as a conceptual umbrella, suggesting that the subsumed disorders are of the same type. In DSM-IV, the SDD umbrella is not used explicitly, but the notion of a discrepancy is present in the categories of Learning Disorders, Motor Skill Disorder and Communication Disorders, suggesting a close relationship between these disorders. One of the advantages of the use of SDD as a unifying concept is that it contributes to the standardisation of the description of the various disorders. However, based on reviews of research regarding the reliability and validity of the SDD categories, we argue that the application of a unifying SDD concept has been premature. For each of the categories for disorders in scholastic skills, language, speech and motor coordination, the notion of a discrepancy between observed and expected level of development should be elaborated and tested more thoroughly, before SDD can be used as a unifying concept in classification.