Despite the plethora of SLA research conducted on the acquisition of the definite marker in noun phrase configurations in L2 Arabic and English (e.g., Sarko, 2007; Master, 1997; Collier, 1987; Anderson, 1984; Kharma, 1981), there is as yet no definitive description of how noun phrases are acquired and why errors persist after advanced stages in L2 learning. Results, as shown by Butler (2002), are inconclusive, and the primary causes of difficulties in the acquisition of the definite marker in noun phrase configurations remain unclear. Recently, the internal syntax-semantics interface (Cuza & Frank, 2011; Montrul, 2010; Tsimpli & Sorace, 2006; Sorace, 2003, 2004) and the specificity-definiteness distinction (Ionin, 2003; Ionin et al., 2004; Ionin et al., 2008) have been considered as appropriate frameworks for exploring the acquisition of noun phrases and other structural features. The structure of noun phrase configurations in Arabic and English offers a complex interface between form and meaning for L2 learners with multiple cases of matches and mismatches between specificity and definiteness. In this three-article dissertation project, two of which were conducted in a bidirectional methodological framework with L1 Arabic-L2 English and L1 English-L2 Arabic learners, I explored the acquisition of three cases of noun phrase configurations. In the first study, I investigated the acquisition of plural noun phrase configurations that carry generic and specific readings at the initial state of L2 learning. Using three data collection instruments: written translation; error detection and correction; and forced choice elicitation, I tested the predictions made by the Interface Hypothesis (IH) and the Full Transfer (FT) Hypothesis. Results showed that L2 learners in both directions tend to transfer noun phrase configurations from L1 into L2, a result that I took to support the FT hypothesis. In addition, it took L1 English-L2 Arabic learners two years of instruction to recover from this L1 effect. The second study aimed at confirming the result of the first study, but in the acquisition of the definite marker in generic singular noun phrase configurations in the L1 English-L2 Arabic direction. The behavior of generic singular noun phrases in L2 Arabic offers a good testing ground since it has numerous similarities and differences with English. Two conditions were established: a matching condition and a mismatching condition. Both conditions were tested in the L1 English-L2 Arabic direction. Results showed a similar pattern to the one recorded in the first study. Typological proximity and distance were found to be important determiners of language acquisition of the in/definiteness configurations of singular noun phrases. In the third study, I shifted to the exploration of a more complex type of noun phrases; namely the definite Iḍāfah construction in Modern Standard Arabic (MSA) and its equivalent noun phrase configurations in English in the two directions. I started with a common difference between MSA and English. Whereas in MSA there is a canonical configuration in terms of head-complement ordering and head-complement definiteness, English is tolerant of more than one permissible configuration. I operationalized the acquisition of these noun phrases in terms of head-complement ordering and head-complement definiteness. Results showed a clear effect of L1 transfer in both directions; knowledge of L1 noun phrase configurations acts as the initial step in L2 learning. I concluded that both communities of L2 learners face problems that vary according to the L1 noun phrase configuration at hand. However, in the L1 English-L2 Arabic direction, learners reached a satisfactory level of performance in the Iḍāfah construction after two years of instruction. I approached this finding as a result of intensive Focus-on-Form Episodes (Loewn, 2005) that the Iḍāfah construction receives in MSA instruction. At the conclusion of this research project I highlighted some implications for the second language acquisition and teaching of noun phrases. The overall results were couched in a broader perspective that characterizes the initial state of L2 learning of noun phrases in + article and – article languages, the effects of typological proximity and distance, and the effects of Instructed SLA. For the pedagogical implications, I called for the integration of the semantics of the definite marker while presenting noun phrases in textbooks. I also recommended the use of explicit instruction and structured-input activities (VanPatten, 2004; Marsden & Chen, 2011) as effective pedagogical tools that foster form-meaning mapping in the acquisition of L2 Arabic and English noun phrases.