It has been known since at least the 1960s that small changes in pupil diameter in response to a mental task are indicative of processing effort associated with this task. More recently, with the advent of modern eye-trackers, which also measure the pupil diameter, pupillometry has been "rediscovered" by language researchers and the method has since been used in many different subdisciplines of linguistics. This article gives a nonexhaustive overview about recent linguistic research with the purpose of introducing researchers in the field of second language acquisition (SLA) to pupillometry. In addition, the article discusses things to consider when designing an experiment and how pupil data can be analyzed. The range of possibilities in which pupillometry can be used in experimental SLA research makes it a welcome addition to other online methods such as eye-tracking and event-related potentials.