This dissertation investigated how the number systems of sign languages (SLs) of deaf people are similar to and different from the number gestures of hearing cultures and the number systems of spoken languages. Number representation in SLs was investigated here in its typological, morphological, and processing aspects. Additionally, the thesis explored the effects of early language deprivation on the acquisition of number, since it is common in the deaf community (90% of deaf children are born in hearing families, do not learn SL from their parents).Culturally specific number gestures are often used by language speakers to indicate number. While SL numeral systems share the same articulatory and perceptual systems, SLs are linguistic systems, unlike number gestures. In the first study of the dissertation, a corpus of numeral systems in 82 SLs was analyzed to investigate how they are similar to and different from spoken languages number systems and number gestures, and to discover the modality-specific, systematic properties of SL numeral systems, their use of iconicity, and the global distribution of one- and two-handed numeral systems. The second study investigated how the iconic two-handed numeral system in Russian SL interacts with the non-concatenative morphology of the language discovered some new phonological constraints in sign language. The number systems of SLs are linguistic, but also visual – a property they share with Arabic digits. Using a Number Stroop Test, the third study revealed that the processing of number lexemes in American SL (ASL) is similar to that of spoken language number lexemes rather than Arabic digits. Both number formats are affected by early language deprivation in terms of processing speed. However, automatic magnitude representation still can be formed despite delayed language exposure. Finally, the linguistic representation of number extends beyond the number systems to grammatical number marking. The last study investigated the impact of early language deprivation on the acquisition of plural classifier constructions, which are a complex morphological means of number marking in ASL. It revealed that late first language learners prefer morphologically simpler strategies over classifiers and make errors specific to their language acquisition background.