The influence of second language proficiency and length of formal training in interpretation on simultaneous interpreting (SI) performance and working memory was examined in Mandarin–English student interpreters with one year ( n = 11) or two years of formal training in interpretation ( n = 9) and in 16 Mandarin–English untrained bilingual controls. SI performance was significantly better in Year 2 than in Year 1 student interpreters, and in Year 1 interpreters relative to bilingual controls. SI performance was also better in advanced L2 users and in high-memory span individuals, whether trained or not in SI. Both Year 1 and Year 2 students outperformed bilingual controls in L1 and L2 reading span. Although Year 2 students tended to show higher working memory span than Year 1 students, the difference was not significant. Finally, working memory span was higher in individuals with greater L2 proficiency. It is concluded that differences in language proficiency may underlie observed differences in both interpreting performance and working memory and that language processing skills (rather than working memory) may be enhanced by formal training in interpreting.