This thesis aims to detect and describe some determinants of the emergence, functioning and implications of institutional competition within the EU. Starting from elements in existing literature, an analytical framework is built up to examine the roles of, inter alia: (1) the channels of competitive interaction, (2) legal actors’ incentives to compete or cooperate, (3) the allocation of choice among different types of regulatees, and (4) the legal norms or governance tools in place, in shaping competitive interactions. These factors (as pieces of a puzzle) are examined through the description of competition in three legal fields: company, securities, and competition law. The descriptions of these three fields, their comparison, and some extrapolations reveal interesting linkages among the different determinants of competition, four examples: (1) & (2) The channels of competitive interaction are likely to influence which legal actors will enter the competition. Legal actors’ incentives to enter into competition can be best grasped through examining their institutional / organisational arrangements such as financing, independence, etc. Given these, the channel of competition, the 'rules of the game' and governance design will greatly determine whether actors will have incentives to compete. (3) The allocation of choice among different types of regulatees may determine the scope of inter-stakeholder externalities. But more importantly, we can make some predictions about the dynamics of such externalities based on the market failure that regulatory intervention aims to target and parties' principal-agent problems; adding to this that regulatees with choice are not necessarily a homogeneous group and actors can have different strategies towards legal arbitrage. (4) The legal tools or governance mechanisms will not only have an impact on legal actors' propensity to compete or cooperate, but also on legal diversity, incentives for innovation, cross-border externalities and market integration - through impacting switching costs across legal regimes.