The increasingly popular term of Fintech captures the accelerated and multidimensional transformation of contemporary financial markets driven and enabled by digital technologies and financial innovation. Although financial regulation is accustomed to grapple with a dynamic, highly changing, and demanding market, and the technological progress has been an important force in the transformation of finance, the vertiginous pace of technological innovation and the unpredictable effects of its application on a large scale, the complexity and the breadth of the phenomenon, and the disruptive potential of the variety of innovative models represent unprecedented challenges for financial regulators and supervisors. Therefore, articulating a balanced, timely and effective regulatory response to Fintech challenges requires first to understand all the dimensions of such transformation, the emerging models, the technological developments and its applications, and the innovative products and services. This book, prepared by the author under the Legal Research Programme 2018 sponsored by the European Central Bank, devises a taxonomy to better comprehend, assess, and systematize the impact of digital innovation on financial markets on the basis of an original theory: the layers of digital financial innovation (architecture and structures, product and services, market players). This theory enables to identify, classify and categorize the challenges for regulation and supervision. Risks and benefits differ in each layer. After the analysis of concrete examples to apply the theory (robo advisers, crowdfunding platforms, aggregators and comparators, social trading and copy trading), the author elaborates a framework (based on three variables) to assess the soundness and test the resistance of the current regulatory system and guide the legal response to digital challenges. Finally, the book concludes with a set of principles and strategies to guide an effective, agile, and opportune regulatory response to Fintech.