Advancedness--high-level language use sophisticated enough to adapt to a variety of sociocultural contexts--encapsulates essential elements of L2 learners’ identity. Advanced proficiency, on the other hand, refers to learners’ communicative ability as determined by formal assessments via pre-established criteria (Brown, Thompson, & Cox, 2021). In addition, the term ‘advanced’ has been erroneously defined by non-experts as being ‘fluent’ or ‘bilingual.’ Conflicting definitions reflect varying perspectives among professionals involved in language programs, which could hinder the establishment of curricula and appropriate assessment metrics. After reviewing the most commonly-used assessment scales (ACTFL, CEFR, and ILR), the chapter focuses on what research has found regarding sociocultural and psycholinguistic individual differences, such as study abroad as well as extracurricular habits and behaviors, pre-tertiary education, age of onset, aptitude, motivation, perseverance, and L2 grit. Other facilitating factors are also mentioned, such as equity in accessing extracurricular opportunities; better academic advising, and acknowledgment of the learners’ construction of an L2 identity.