Specific language impairment (SLI) is one of the most common childhood disorders, affecting 7% of children. These children experience difficulties in understanding and producing spoken language despite normal intelligence, normal hearing, and normal opportunities to learn language. The causes of SLI are still hotly debated, ranging from nonlinguistic deficits in auditory perception to high-level deficits in grammar. Here, we show that children with SLI have poorer-than-normal consonant identification when measured in ecologically valid conditions of stationary or fluctuating masking noise. The deficits persisted even in comparison with a younger group of normally developing children who were matched for language skills. This finding points to a fundamental deficit. Information transmission of all phonetic features (voicing, place, and manner) was impaired, although the deficits were strongest for voicing (e.g., difference between͞b͞and͞p͞). Children with SLI experienced perfectly normal ''release from masking'' (better identification in fluctuating than in stationary noise), which indicates a central deficit in feature extraction rather than deficits in low-level, temporal, and spectral auditory capacities. We further showed that speech identification in noise predicted language impairment to a great extent within the group of children with SLI and across all participants. Previous research might have underestimated this important link, possibly because speech perception has typically been investigated in optimal listening conditions using non-speech material. The present study suggests that children with SLI learn language deviantly because they inefficiently extract and manipulate speech features, in particular, voicing. This result offers new directions for the fast diagnosis and remediation of SLI.phonetic deficit ͉ auditory deficit ͉ speech intelligibility ͉ masking noise ͉ specific language impairment M any children experience unexpected difficulties in understanding and producing spoken language despite normal intelligence, normal hearing, normal opportunities to learn language, and in the absence of any obvious neurological problems (for review, see ref. 1). This disorder is typically called specific language impairment (SLI). In the past years, research on SLI has experienced a growth of interest partially because it has become clear that more children than initially thought show language learning difficulties. Indeed, recent epidemiological studies estimate the incidence of SLI to be Ϸ7.4% in a population of monolingual English-speaking kindergarten children (2). Children with SLI exhibit deficits in several aspects of language, including phonology, morphology, and syntax (1, 3). One of the hallmarks of SLI is a deficit in the use of function morphemes (e.g., the, a, and is) and other grammatical morphology (e.g., plural -s, past tense -ed). Children with SLI also are at high risk for subsequent literacy problems (4).The causes of SLI are still hotly debated. Current theories of SLI fall into two categ...