How cats became the undisputed mascot of the internet. The advertising slogan of the social news site Reddit is "Come for the cats. Stay for the empathy." Journalists and their readers seem to need no explanation for the line, "The internet is made of cats." Everyone understands the joke, but few know how it started. A Unified Theory of Cats on the Internet is the first book to explore the history of how the cat became the internet's best friend. Internet cats can differ in dramatic ways, from the goth cats of Twitter to the glamourpusses of Instagram to the giddy, nonsensical silliness of Nyan Cat. But they all share common traits and values. Bringing together fun anecdotes, thoughtful analyses, and hidden histories of the communities that built the internet, Elyse White shows how japonisme, punk culture, cute culture, and the battle among different communities for the soul of the internet informed the sensibility of online felines. Internet cats offer a playful-and useful-way to understand how culture shapes and is shaped by technology. Western culture has used cats for centuries as symbols of darkness, pathos, and alienation, and the communities that helped build the internet explicitly constructed themselves as outsiders, with snark and alienation at the core of their identity. Thus cats became the sine qua non of cultural literacy for the Extremely Online, not to mention an everyday medium of expression for the rest of us. Whatever direction the internet takes next, the "series of tubes" is likely to remain cat-shaped.