This book helps readers understand the widely documented distortion in the portfolio choice of individual investors toward proximate firms - the proximity bias phenomenon. First, it recapitulates the fundamentals of modern portfolio theory. It then goes on to describe and demonstrate different approaches on how to measure proximity bias and identifies and examines potential motives and reasons for such a bias. In addition, the book presents new analysis on the financial effects of individual investors' proximity bias, explaining and contributing with possible policy implications on their portfolio distortion. This book will be of interest to students and researchers, as well as decision-makers in business firms and households.