The personalization of advertising offers firms tremendous potential. If done right, firms can address consumers with more relevant ads, leading to more positive consumer responses. Nevertheless, firms are struggling with how to design personalization strategies and face the challenge to correctly assess advertising effectiveness. With this research, we advance the understanding of advertising personalization and its implications for firms, consumers, and ad platforms. With the help of a large-scale field experiment, we present evidence for how firms should design their personalization strategies. We find that high levels of personalization specificity pay off for firms. At the same time, socially targeting personalized ads, where nam...