The aim of this dissertation is to investigate the contribution of lexical factors that affect second language (L2) lexical access, such as size of L2 mental lexicon, lexical frequency, and number of competitors. It introduces and explores an additional L2-specific dimension that plays a differential role in L2 lexical access, which is the degree of familiarity with the L2 lexical item, in particular, familiarity with its phonological form as it maps onto its meaning. The current thesis focuses on this factor's main consequence, which is the underspecification of the phonological representation of less-known words in the L2 mental lexicon. The combination of traditional lexical factors with the proposed L2-specific lexical factor makes it possible to propose an L2-specific model that accounts for the interactions not found in L1 lexical access mechanisms. The Second Language Lexical Access Model (SLLAM) proposed in the dissertation incorporates L2 specific factors, such as the underspecification of phonological representations and the proficiency-defined size of the mental lexicon, and makes predictions about the process of lexical access in L2. The dissertation compares lexical access mechanisms in three groups of subjects, two of which are L2 learners of Russian at different stages of acquisition (Intermediate learners and Advanced learners), and uses novel empirical evidence from five behavioral experiments: lexical decision task without priming, lexical decision task with phonological priming, lexical decision task with semantic priming, lexical decision task with pseudo-semantic priming, and a translation task. The results of the experiments are discussed in light of the proposed SLLAM model. The dissertation argues that the majority of the observed results can be accommodated by the assumptions made by SLLAM, compatible with the postulated underspecification at the lexical level of L2 phonological representations. Moreover, the study concludes that some of the L2-specific lexical access mechanisms, commonly attributed to a lack of semantic links within the lexicon, may be more parsimoniously explained as resulting from phonological underspecification as well.