TV audience measurement involves estimating the number of viewers tuned into a TV show at any given time as well as their demographics. First introduced shortly after commercial television broadcasting began in the late 1940s, audience measurement allowed the business of television to flourish by offering networks a way to quantify the monetary value of TV audiences for advertisers, who pay for the estimated number of eyeballs watching during commercials. The first measurement techniques suffered from multiple limitations because reliable, large-scale data were costly to acquire. Yet despite these limitations, measurement standards remained largely unchanged for decades until devices such as cable boxes, video-on-demand boxes, and cell phones, as well as web apps, Internet browser clicks, web queries, and social media activity, resulted in an explosion of digitally available data. TV viewers now leave digital traces that can be used to track almost every aspect of their daily lives, allowing the potential for large-scale aggregation across data sources for individual users and groups and enabling the tracking of more people on more dimensions for more shows. Data are now more comprehensive, available in real time, and cheaper to acquire, enabling accurate and fine-grained TV audience measurement. In this article, I discuss the evolution of audience measurement and what the recent data explosion means for the TV industry and academic research.