Eye-tracking, an innovative research method that measures the movements of the eyes, has recently gained much attention in the areas of second language learning and assessment. The advantage of using eye-tracking is that it provides a direct measure of cognitive processes in a natural condition. The present paper provides a systematic review of what eye-tracking is, both in theory and practice, how it works from preparation to interpretation of the results, and what strengths and weaknesses it possesses. For this, an existing eye-tracking study has been used as a sample to demonstrate what needs to be considered in the stages of data collection, data analysis, and interpretation. The present paper also provides some empirical data and findings from the sample study as examples to demonstrate how the eye-movement data are computed and analyzed to explain a participant’s viewing behavior. By introducing different types of eye-movement data, the paper illustrates how different measures and forms of eye-movements provide discrete meanings to explain participants’ cognitive processes. This study concludes by providing a few practical suggestions to improve the validity of eye-tracking studies for effective uses of the method.