Prelingually deaf children unavoidably make deviations in production although they possess an abstract phonological system. The systems differ from those of normal speakers due to the fact that the phonological systems of deaf children are built up through vision, tactilation and maybe some residual hearing. Traditionally, assessments of deaf speech describe what a deaf child is not capable of articulating through conventional error-analyses. Hence, only speech sounds that the child never articulates correctly are treated in the speech clinic. The sounds that the child articulates correctly are disregarded and also taken for granted to be used correctly. However, it has been shown that even if a child knows how to articulate a sound correctly this does not imply that the usage is correct in his spoken language. Furthermore, many of the articulatory deviations made by deaf children are in fact attempts to realize phonological contrasts. Hence, a conventional error-analysis provides no information about a deaf child's usage of his or her productive knowledge and pays no attention to whether a deviant articulation might signal a "correct" contrast. It is more appropriate to base a speech-training programme on a phonological assessment that determines what the speech does express.