The proper functioning of the EU financial market is protected by public actors, both national and supranational, who are responsible for rulemaking and for supervision of investment firms and other private actors. At the same time, the effectiveness of the EU legal system requires vigilance from private actors such as investment firms and their clients, who may invoke their rights before national authorities and courts. This means that investment firms have a dual role within the system, as subjects of control and enforcement, as well as agents in the maintenance of the rule of law. This book brings together a group of scholars with expertise in different legal disciplines, but a shared interest in the EU internal market and its development. It is intended to integrate a modern study of the form and function of EU rulemaking in the internal market after the financial crisis with an evaluation of core aspects of rulemaking in the financial market and, in this way, to provide a cross-cutting treatment of EU law. The book focuses on the regulatory framework in MiFID II and MiFIR, and the following thematic questions: what are the legal mechanisms for accountability, and what is the role of investment firms in the operation of those mechanisms?; what are the implications of the answers to the previous question for EU law and the EU legal system?; how do the findings contribute to the understanding of the concept of accountability?