The different perceptual and productive systems of signed and spoken languages may result in differing constraints on the nature of linguistic processing. Sign languages present a natural opportunity to explore how the modality in which a language is expressed crucially affects the psychological mechanisms required to decode and produce the linguistic signal. One goal of this chapter is to examine what aspects of language processing and production may be universal and what aspects are affected by the particular characteristics of audition versus vision or by the specific constraints on gestural versus vocal articulation. Another aim is to provide a review of what is currently known about the psychological processes involved in sign language comprehension and production. We compare sign language processing to speech, rather than to reading, because unlike written text, which can be characterized as “visual language,” sign language consists of dynamic and constantly changing forms rather than static symbols. In addition, neither sign language nor spoken language comes presegmented into words and sentences for the perceiver. Finally, in contrast to written language, sign and speech are both primary language systems (see chap. 7), acquired during infancy and early childhood without formal instruction (see chap. 5).