“Book Descriptions: When measured by assets under management, the majority of equity investing is fundamentally driven. Portfolio managers identify investment opportunities and translate them into portfolios. Although the portfolios may take many shapes, depending on the mandates and constraints of the funds, they are constructed according to a set of shared principles. Over the course of the past 30 years, the industry has matured and become more sophisticated; in the process, the principles have evolved and refined. This process of continuous experimentation and selection has happened by and large in the investment firms, and has been driven by their most skilled practitioners, with a healthy input from finance academics. Factor risk models play a central role, but not in the same way they do in the "factor investing" (or "smart beta") investment style. Rather than as a way to identify investable anomalies, factor modeling offers an extremely flexible and insightful framework for risk quantification, portfolio analysis, and portfolio construction. Being developed by trial-and-error by professionals in an industry that does not value open communication, this body of knowledge has been transmitted primarily via a system of apprenticeship. Veteran risk managers tutor junior ones, who work with junior portfolio managers, who will demand that their own team members understand the principles, and so on. To use a term coined by Bertrand Russell, this is "knowledge by acquaintance": experienced and direct. Its counterpart is "knowledge by description". Specialized knowledge by acquaintance does not scale, and is fragile: Leonardo fruitfully apprenticed with Verrocchio, and became a great technical innovator; but his knowledge died with him, because he had no disciples.” DRIVE