The three studies reported in this thesis investigated the contributing factors of L2 writing among adult ESL learners in the academic setting. The major purpose of this research was to explore the relationship between L2 proficiency, writing strategies, writing attitude, writing errors and L2 writing performance. This thesis aimed to provide insights for the contributing factors that are predictive of L2 writing performance in adult ESL learners, studying in English and non-English dominant settings. Study 1 (reported in Chapter 3) focused on determining the appropriate measures for investigating the individual factors of writing performance; particularly learners’ writing strategies, learners’ second language proficiency, first language (L1) interference and their relation to writing performance. Thirty-one intermediate students of L2 served as participants. A measure of vocabulary size and a writing strategy questionnaire were administered to the students. Findings in this study indicated that most of the participants’ planning strategies were limited to having a mental or written plan whereas over half of the respondents reported that they always start with an introduction and were more likely to stop drafting after a few sentences. In terms of drafting strategies, it was found that most respondents reread what they had written to get ideas on how to continue but did not go back to their outline to make changes in it. With regard to L1 use, a majority of participants do not write bits of text in their native language. Nevertheless, quite a number of participants indicated that they would write in their L1 if they don’t know a word in English. Findings in this study also suggested that participants’ biggest concerns were related to grammar and vocabulary, which resulted in them making surface level changes and checking. An overall analysis of participants’ writing output and responses from the questionnaire also provided important insights to the improvement of the measures. The revision process included rewording and rephrasing ambiguous items, removing irrelevant items from the questionnaire and restructuring the writing task for the next study. In Study 2 (presented in Chapter 4), a follow-up study was conducted to examine L2 writers’ proficiency level, writing attitude, writing errors and writing strategies in an English-dominant setting. Nine research questions were designed to guide the study framework and gather specific data regarding the research aims. A writing measure, vocabulary tests and a questionnaire were administered to the students. Findings from Study 2 indicated that L2 proficiency, particularly vocabulary size, was related to writing performance. In addition, it was also discovered that L2 writers who performed poorly were prone to performing writing strategies related to surface level checking. Therefore, it was concluded that linguistic barriers in L2 affect both writing performance and students’ ability in applying the effective strategies in writing. Apart from that, Study 2 also found that the use of L1 and translation into L2 was associated with lower writing performance. Additionally, Study 2 found that pronoun, word and sentence errors were the most prevalent errors among ESL students. A possible reason for this is because L2 students need to work with two languages while writing, mainly the grammar rules in English which are not found in their L1 as well as their own native language. Thus, L2 students face the challenge of working out English grammar rules while writing. Overall, findings in this study suggest that prevalent writing errors in English may be a sign of L1 interference and that as the use of L1 increases, writing performance decreases. In Study 3 (reported in Chapter 5), the role of proficiency level, writing attitude, writing errors and writing strategies was explored by measuring the relationship between writing attitude scores, errors in writing, strategy use and essay scores. Additionally, the role of L2 proficiency in writing performance was also investigated by assessing the relationship between vocabulary size scores, writing errors and writing performance. Findings from Study 3 revealed unexpected findings with regard to the relationship between L1 use and writing performance among the three sample groups. L1 use was found to be correlated with writing performance for Group A but not Groups B and C. It was argued that L2 writers of different L2 proficiency level and academic experience may have different orientations of L1 use. Further work on the impact of L1 use on L2 writing will be needed in order to provide insights into this area. With regard to writing errors, a relationship between errors and writing performance was reported. It was found that subject verb agreement error appeared to be a common factor for the three groups in the study that was related to writing performance. In addition, errors were also significantly correlated with L2 proficiency, suggesting that as L2 proficiency increased, errors decreased. Overall, Study 3 argues for the importance of developing and enhancing learners’ L2 proficiency to reduce errors and improve learners’ writing performance. Additionally, Study 3 also argues for the need to emphasize effective writing strategies in the ESL writing classroom.