Learning a second/foreign language can be an arduous process during which the language learner may encounter many challenges both from the context and from within. To maintain the learning process and to succeed, it seems that the learner needs resilience - the ability to bounce back from adversity. Although success in second/foreign language learning has been claimed by SLA researchers to be attributable to various contextual and individual factors, resilience has received scant attention in the field of second language acquisition. This study explores the concept of resilience to shed light on the phenomenon of success in foreign language learning despite challenges and difficulties. It aims to conceptualise foreign language learner resilience in the context of English teaching and learning at the university level in Vietnam. The study is guided by the overarching research question What does foreign language learner resilience look like? This study explored foreign language learner resilience as a complex dynamic system. Accordingly, it took the notions of self-organisation of complex dynamic systems and retro-diction in researching complex dynamic systems as the basis for designing the research. The foreign language learner resilience system, hence, was explored retrospectively by identifying a selection of people who seemed to typify resilient learners and the study worked backwards to explain how they had become so. The study employed a qualitative research design which included focus group discussions and semi-structured interviews, sequentially linked with each other, as the two main data collection methods. Three focus group discussions were conducted with teachers of English at a university in Vietnam (N=13) to identify typical resilient language learners seen as possible patterns in the system. Based on the focus group discussion data, questions for semi-structured interviews were developed for the second phase of data collection which aimed to identify the components of the system. Thirty students who self-identified as resilient English learners voluntarily participated in the semi-structured interviews. They included first-year students (N=17) and fourth-year students (N=13) from the same university. Findings indicated that foreign language learner resilience is a complex dynamic system, composed of contextual and individual factors emerging from the interactions between the students and aspects of three contextual dimensions (community/society, institution and family), across their English learning trajectories. Core to the system are motivation, emotion, agency, autonomy, perseverance and optimism evolving and fluctuating in association with their interactions with the above contexts. The system is characterised by the nonlinear interactive mechanism between factors both detrimental and conducive to the language learning process. The findings of this study attest to the conceptualisation of resilience as a process in mainstream psychological research. Given the scant research into the concept in the SLA field, this study provides a new and deeper understanding of resilience in second/foreign language learning. More importantly, its findings help integrate a disparate set of contextual and individual factors influencing second/foreign language learning success. As the study presents a new concept to the context of foreign language education in Vietnam, it provides teachers and administrators with insights into the difficulties and challenges Vietnamese EFL learners at the university level might face and the resources they can draw on in the face of adversity or stress. Drawing on the findings, it is suggested that teacher-student rapport should be enhanced to contribute to generating a synergy of motivation, positive emotions, autonomy and agency, perseverance and optimism that help the learners withstand challenges and difficulties to succeed in foreign language learning.