This DPhil thesis explores judicial activism in economic telecommunications regulation. It enquires into what factors explain its occurrence. Two debates in different strands of literature - and gaps in them - inspire the research. A first one is that various theories attempt to explain judicial activism, but no general agreement can be found on what factors actually have key explanatory power. The second gap is that the literature on regulation usually considers judicial actors as having a role in the enforcement or accountability stages of the regulatory process but largely ignore the role that they could have in the development of regulation. Empirically, I analyse judicial activism in relation to economic telecommunications regulation through an exploratory comparative study. The two countries, Chile and the UK, are chosen because they have similarities and differences that made them interesting for a comparative project, such as having different legal systems (common and continental law) and having a different political history but similar approaches to regulating telecommunications (privatisation and liberalisation). Through a critical analysis of interviews and of final decisions made by judicial actors in both countries, I explore whether the factors the relevant literature argues explain judicial activism actually do explain judicial activism in the two case studies, whether other factors not found in the literature are important in explaining judicial activism, and - most importantly - how these factors interact. Through the analysis based on the case studies I reach three main conclusions. First, that judicial activism is a jurisdiction specific phenomenon and different factors are found to explain it in each case. Second, the different factors that explain judicial activism - I argue - play different roles. Some are argued to act as enabling factors of judicial activism while others explain judicial activism in an immediate manner, which I call direct drivers. Finally, I argue that the findings of this thesis contribute to debates in different strands of literature, such as legal theory, and specifically descriptive accounts of adjudication.