Developmental disorders of speech and language (DDSL) may be subclassified into specific DDSL (SDDSL) and DDSL associated with language-relevant comorbidities (DDSLC). For SDDSL (synonym: primary or specific language impairment, SLI), other language-relevant comorbidities or conditions are excluded. They are mainly caused by genetic factors. For DDSLC (synonym: secondary language impairment), either the comorbidity is the only reason for the language disorder or additional factors are present that are also causal of SDDSL. Comorbid disorders may be persistent (e.g. sensorineural hearing loss) or intermittent (e.g. otitis media with effusion). Hearing loss, syndromes, pervasive disorders such as autism-spectrum disorders, other developmental disorders and intellectual disability are important comorbid disorders affecting speech and language development. Non-pathological sociogenic or environmental factors, such as adverse social conditions, may cause abnormalities in speech-language development and have to be discerned from DDSL. Only DDSL needs a language therapy, but both DDSL and sociogenically caused language abnormalities benefit from more and high-quality language input.