This chapter discusses the researchers who are interested in using pupil diameter as a data input in their research but have little or no prior experience. It explores the modern method of measuring pupil diameter, the common environmental control and experimental design that need to be considered, post-experiment data analysis, and the difficulties in interpreting pupil dilation results along with some solutions. The chapter deals with some examples of actual pupil dilation research. Experimental design for pupil dilation research differs from the usual eye-tracking experiments for several reasons. There are some common static measurements that are being reported regarding the trial pupil data: mean pupil dilation, peak pupil dilation, and latency to peak. The main interpretive challenge is to isolate the exact cause of the pupil dilation. In addition to combining measurements, interpretation of pupil dilation can also be aided by relevant theory that explains behavior.