Our understanding of the mental lexicon, the way meaning is extracted from word forms, is almost entirely built on data from spoken languages. While there is much work demonstrating that in many ways the linguistic structure and psychological mechanisms for processing signed language and spoken language processing are the same, less is known about the signed language mental lexicon. In this dissertation, I examine the structure of the American Sign Language mental lexicon, and the ways meaning can be extracted from the manual/visual signal. In the third chapter of this dissertation I ask whether a single cognitive architecture might explain diverse behavioral patterns in signed and spoken language. Chen and Mirman (2012) presented a computational model of word processing that unified opposite effects of neighborhood density in speech production, perception, and written word recognition. Carreiras et al. (2008) demonstrate that neighborhood density effects in Spanish Sign Language (LSE) also vary depending on whether the neighbors share the same handshape or location. We present a spreading activation architecture that borrows the principles proposed by Chen and Mirman (2012), and show that if this architecture is elaborated to incorporate relatively minor facts about either 1) the time course of sign perception or 2) the frequency of sub-lexical units in sign languages, it produces data that match the experimental findings from sign languages. This work serves as a proof of concept that a single cognitive architecture could underlie both sign and word recognition. In the second chapter I present ASL-LEX, a lexical database for ASL that catalogues more than forty properties about almost 1,000 signs. The database includes, for example, information about each sign's iconicity, phonological make-up, and neighborhood density. I use this information to better understand the structure of the ASL lexicon, the distribution of each of these properties, and the relationships between these properties. This lexical database is the largest and most comprehensive database of ASL, and can be used by researchers to develop experiments and by educators to identify and support vocabulary development. In the fourth chapter, I use ASL-LEX to develop a tightly-controlled study of sign perception. I ask whether neighborhood density and sub-lexical frequency play a role in sign perception, and if the mechanisms of sign perception are affected by early language experience. Eighty deaf participants with varying early language backgrounds completed a lexical decision task. I find that neighborhood density inhibits sign perception in people with low early ASL exposure, but has no effect in people with high early ASL exposure. Location frequency inhibits sign perception in all people, but the effect is stronger in people with low early ASL exposure. This suggests that impoverished access to ASL early in life has lasting consequences for sign perception. (PsycINFO Database Record (c) 2016 APA, all rights reserved)