Pupillometry, the study of the eyes’ pupils, has a rich history, dating back to the 1800s. For example, to appear “dark with desire,” women once used atropa belladonna (deadly nightshade) as a cosmetic, because the atropine dilated their pupils, making them appear more romantically aroused. We now know that this relationship is largely driven by the activity of the sympathetic nervous system; specifically, a small brainstem nucleus known as the locus coeruleus (LC). Because of tight connections between the musculature of the eyes and LC, monitoring the pupils can reveal important insights into brain activity during mental processes. Many of these processes are related to attention and arousal (cognitive or emotional), with the LC controlling mental readiness via secretion of the neurotransmitter norepinephrine. While these complicated neurochemical processes happen in the brain, they are often overtly observable via pupil dilation. Although pupillometry was popular in the 1960s and 1970s, it fell out of favor until experiencing a renaissance approximately 10 years ago. With the advent of new eye-tracking and neural recording technology, measuring (and analyzing) pupil size is now easier than ever. Because all modern eye-trackers use pupil size in the calculation of gaze location, they also provide researchers with moment-by-moment pupil size measures in output files. Although previously considered “extra” data to support gaze location analyses, researchers have begun to conduct eye-tracking studies solely to gain access to pupil size data. These data have been used to study thought processes in many domains, including cognitive science, psychopathologies, business/marketing, security contexts, and the study of addiction. The diversity of interest in pupillometry is matched by the diversity in approaches taken to data collection, analysis, and interpretation. To date, there exists no book or tutorial review devoted specifically to ensuring that researchers carry out rigorous and reproducible work across these varied domains. Modern Pupillometry: Cognition, Neuroscience, and Practical Applications fills this gap by exploring the history, neuroscience, and methodological considerations of pupillometry research within and beyond psychology.