Index funds are the most widely influential investment vehicles available. They have revolutionised investing by saving millions of people billions of dollars in fees that would otherwise have gone to fund managers. It is no exaggeration to say that the rise of passive investing is probably one of the most consequential financial inventions of the past half-century, by rewiring markets and reshaping the finance industry. Yet some detractors say that index investing is an insidious disease and, with their rapid expansion and grip on the financial market, index funds may have cataclysmic consequences that we aren't even aware of yet. What might the socio-economic risks of wide-spread passive investing be? What are the longer-term consequences to capitalism? And what does the future look like for the investment landscape? Through exclusive interviews with key industry giants, Robin Wigglesworth, the Financial Times' New York-based markets editor, reveals the thrilling and untold history of the revolutionists behind the invention of index funds and investigates one of the most pressing financial uncertainties of our time.