This book analyses search engines as indexing systems: structures of social memory, designed to organise information access when knowledge is stored in archives. It views indexing systems as a highly improbable outcome of socio-cultural evolution, and charts the main stages of this social process through the impact of printing on knowledge organisation, the rise of mechanical memory, the practice of tagging, and contemporary strategies of de-indexing. In the process, the book sheds new light on the underexplored sociological question "Where do search engines come from?"