How do the Internet and social media affect political outcomes? We review empirical evidence from the recent political economy literature, focusing primarily on work that considers traits that distinguish the Internet and social media from traditional off-line media, such as low barriers to entry and reliance on user-generated content. We discuss the main results about the effects of the Internet in general, and social media in particular, on voting, street protests, attitudes toward government, political polarization, xenophobia, and politicians' behavior. We also review evidence on the role of social media in the dissemination of fake news, and we summarize results about the strategies employed by autocratic regimes to censor the Internet and to use social media for surveillance and propaganda. We conclude by highlighting open questions about how the Internet and social media shape politics in democracies and autocracies.