The phonological system of a sign language comprises meaningless sub-lexical units that define the structure of a sign. A number of studies have examined how learners of a sign language as a first language (L1) acquire these components. However, little is understood about the mechanism by which hearing adults develop visual phonological categories when learning a sign language as a second language (L2). Developmental studies have shown that sign complexity and iconicity, the clear mapping between the form of a sign and its referent, shape in different ways the order of emergence of a visual phonology. The aim of the present dissertation was to investigate how these two factors affect the development of a visual phonology in hearing adults learning a sign language as L2. The empirical data gathered in this dissertation confirms that sign structure and iconicity are important factors that determine L2 phonological development. Non-signers perform better at discriminating the contrastive features of phonologically simple signs than signs with multiple elements. Handshape was the parameter most difficult to learn, followed by movement, then orientation and finally location which is the same order of acquisition reported in L1 sign acquisition. In addition, the ability to access the iconic properties of signs had a detrimental effect in phonological development because iconic signs were consistently articulated less accurately than arbitrary signs. Participants tended to retain the iconic elements of signs but disregarded their exact phonetic structure. Further, non-signers appeared to process iconic signs as iconic gestures at least at the early stages of sign language acquisition. The empirical data presented in this dissertation suggest that non-signers exploit their gestural system as scaffolding of the new manual linguistic system and that sign L2 phonological development is strongly influenced by the structural complexity of a sign and its degree of iconicity.