Additional language (L2/Ln) research largely focuses on learners whose first languages are spoken and who are learning additional spoken languages. In the past few decades, sign languages have become increasingly popular for hearing students in schools. These students must not only learn the vocabulary and grammar of sign languages but also manage a different modality (that is, the channels of production and reception of language) than their first language. This raises questions about the role of both language and modality in the L2/Ln learning of sign languages for non-signers. In other cases, deaf and hearing signers of a sign language learn a different sign language, raising questions about L2/Ln learning in the signed modality. This "Special Issue" consists of empirical contributions and a conceptual review article that examine how language and modality shape the learning of sign languages as additional languages. Theoretical issues concerning learning a sign language as another language are discussed.