Simultaneous interpreting is a complex skill in which language comprehension and production take place at the same time in two languages. In this study, we examined performance on basic language and working memory tasks that have been hypothesized to engage cognitive skills important for simultaneous interpreting. The participants were native Dutch speakers proficient in English as a second language. We compared the performance of trained interpreters to bilingual university students (Experiment 1) and to highly proficient English teachers (Experiment 2). The interpreters outperformed the university students in their speed and accuracy of language performance and on their memory capacity estimated from a set of (working) memory measures. The interpreters also outperformed the English teachers, but only on the memory tasks, suggesting that performance on the language tasks was determined by proficiency more than cognitive resources. Taken together, these data point to (working) memory as a critical subskill for simultaneous interpreting.