Starting from the work of Levin and Rappaport Hovav (1995, 2005), this thesis gives an account of the lexical and morphological properties that are relevant to the syntactic expression of causative and inchoative events in Arabic and English, and reports a bidirectional study of how second language (L2) learners of these two languages acquire the different properties involved. It is argued that the two languages differ in both types of property. Given this, by comparing L1 Arabic-speaking learners of L2 English with L1 English-speaking learners of L2 Arabic, it is possible to address questions about the effects of L1 influence and the potential role of innate linguistic knowledge (Universal Grammar) on the development of L2 learners' 'interlanguage' grammars. The Full TransferIFull Access hypothesis of Schwartz and Sprouse (1994, 1996) predicts that the entirety of the L1 lexicon transfers at the initial state. At stages beyond the initial state, L2lexical properties will be acquired, yet L1 influences will persist. Results from grammaticality judgment and forced-choice elicitation tasks conducted with L2 speakers, involving a range of verb types with the potential to express causative/inchoative event meanings, show that low-proficiency learners who are no longer at the initial state nevertheless transfer lexical properties from their L1 into their 'interlanguage' grammars. Results from more proficient L2 learners at later transitional stages show that the two language groups follow different developmental patterns. Arabic learners of English produce overcausativisation and overpassivisation errors before they start to retreat from this overgeneralisation. In contrast, the English-speaking learners of Arabic are more target-like in their use of causatives/inchoatives at all proficiency levels. The implications of these findings for understanding the relative roles that the L1 properties, L2 properties and Universal Grammar play in the acquisition of lexical properties by L2 learners are discussed.