Word recognition is a basic aspect of vocabulary skill, and a critical skill in fluent reading. Native speakers of English can recognize single words in about one tenth of a second. Learners are somewhat slower, but this difference is difficult to measure without sensitive equipment. This chapter describes how we developed a test of word recognition for EFL learners, called Q_Lex. In our approach, words are hidden in nonsense letter strings and this slows recognition speed to a level that personal computers can easily measure. Learners are assessed on the basis of native speakers' reaction time norms. We describe the development and validation of this tool and the measurement principles that underlie it. Especially, we emphasize how we sought to improve its reliability. Finally, we describe an experiment with Q_Lex to investigate learners at different levels of proficiency.