Background Literature Review: Transition markers, as a metadiscourse marker, have been named differently such as internal conjunctions (Halliday & Hasan, 1976; Hyland & Tse, 2004), linking adverbials (Biber et al., 1999), linking adjuncts (Richards & Schmidt, 2010), cohesive ties (Al-Jarf, 2001), discourse connectives (Blakemore, 2002), linkers (Thornbury, 2006), and logical markers (Mur Duen~as, 2009). In the current study, we adopt Hyland's (2005) semantic subtypes of transition markers (i.e., addition, comparison & contrast, and consequence markers). At the textual level, transition markers help create cohesion by showing the logical links between propositions (Cao & Hu, 2014), leading readers to interpret meaning purposefully (Blakemore, 2002), and functioning ideationally by signalling how the writer logically relates different ideas (Hyland & Tse, 2004). Also, students' appropriate use of transition markers semantically can help alleviate the reader's burden of connecting preceding and subsequent content information (Cao & Hu, 2014). Thus, transition markers are important to the quality of L1 Arabic writing (Nasib, 2018) and L2 English writing (Hinkel, 2001). Some previous studies researched transition markers in L1 Arabic (Rabbah, 2016; Abdullah, 2017; Nasib, 2018); other studies investigated transition markers in L2 English of Arab students (Hinkel, 2001; Mohamed-Sayidina, 2010; Al-Rubaye, 2015; Appel & Szeib, 2018; Appel, 2020). Yet, the use of these markers by Arab L2 student writers remains under-researched and problematic (Khalil, 1989; Al-Jarf, 2001; Ahmed, 2010; Hamed, 2014; Alshahrani, 2015; Basheer, 2016; Appel & Szeib, 2018). Yet, investigating transition markers in L1 Arabic and L2 English needs further investigation (Alshahrani, 2015). Previous research highlighted the need for further research to better understand how alternative L1 groups (other than English) use transition markers in their writing (Appel, 2020). Therefore, our current study bridges the gap in the literature by contrastively analysing university students' use of transition markers in their L1 Arabic and L2 English argumentative writing. Therefore, the present study contributes to knowledge by investigating the quantity and variety of transition markers in L1 Arabic and L2 English argumentative writing, written by the same students, across different language proficiency levels (high, average, and low) and gender (154 females and 41 males). It also explores students' metalinguistic understanding of these transition markers in writing. Research Questions: 1. What is the difference (if any) in the overall quantity of transition markers used by L1 Arabic and L2 English university students of different proficiency levels? 2. What is the difference (if any) in the variety of transition markers used by L1 Arabic and L2 English university students of different proficiency levels? 3. What is the gender difference (if any) in students' use of transition markers used by L1 Arabic and L2 English university students? 4. What is the nature of students' metalinguistic understanding of transition markers in L1 Arabic and L2 English university students? Methods: The current study adopts a mixed-methods design, combining a corpus-based methodology with students' metalinguistic understanding. Two corpora were built to investigate the English and Arabic writing of Qatari university students (195 essays each). Students' essays were rated for proficiency in Arabic (9 High, 184 Average, 2 Low) and in English (23 High, 147 Average, and 25 Low. In addition, students' metalinguistic understanding of transition markers was explored through writing conversation interviews with 51 participants. All participants were native Arabic speakers and L2 English speakers. Each participant wrote one essay in Arabic and one in English on two different topics. All ethical issues were addressed (BERA, 2018). Findings: The following findings were revealed. First, no significant statistical differences were found in the quantity of transition markers in students' L1 Arabic and L2 English argumentative writing across the three proficiency levels. Second, no significant statistical differences were found in the variety of transition markers (i.e., addition, comparison & contrast, and consequence markers) across the three proficiency levels. However, (1) two addition markers in Arabic (furthermore [foreign characters omitted] & as well as [foreign characters omitted]) and one addition marker in English (not to mention) were statistically significant; (2) the following comparison & contrast markers in Arabic were statistically significant (the other side [foreign characters omitted], the other team [foreign characters omitted], the first team [foreign characters omitted], on the opposite [foreign characters omitted], at the same time [foreign characters omitted], as an alternative [foreign characters omitted]) and the following markers in English were statistically significant (even though, the same goes for, the first team, the other team; (3) the following consequences markers in Arabic were statistically significant (because [foreign characters omitted], thanks to [foreign characters omitted], so as to [foreign characters omitted], consequences of [foreign characters omitted]) and the following consequences markers in English were statistically significant (Lead, So as to). In reference to gender differences, the following findings were obtained. First, Arabic addition markers are significant at the 0.05 level and favour females compared with males. No significant differences between males and females were found on the comparison & contrast markers and consequence markers. Second, no significant differences between males and females were found for any type of transition marker in English. Third, there are no significant statistical differences in the variables of Addition and Consequences in Arabic and English, as the T-test is not significant. On the other hand, there are differences in Comparison and Contrast markers, as (8.508) shows that T is significant at the 0.001 level in favour of English. The differences in quantity, variety, and gender among the participants might be attributed to factors such as educational, socio-cultural, and L1 transfer into L2 factors. Findings revealed students' metalinguistic understanding of their reasons, importance, specific purposes, and challenges with using transition markers in L1 Arabic and L2 English argumentative writing. Pedagogical and methodological implications are provided.