The new European Financial Supervisory Authorities have received much attention in the literature, particularly due to their exceptional emergency decision-making powers. By contrast, this article explicitly chooses to focus on these agencies' less explored yet equally crucial role: their (quasi-)rule-making responsibilities. While being less striking at first sight than their emergency counterparts, these rule-making powers are considerable, carry significant consequences, and raise some interesting dilemmas and concerns. This article complements the previous contribution by going at a lower level of specification and zooming in on a crucial case for studying rule-making by agencies as the Authorities constitute a culmination of agency rule-making powers, as well as agency powers, more broadly. The article will analyse the Authorities' main (quasi-)rule-making powers and the relevant procedures. It will specifically investigate their role with respect to the adoption of regulatory and implementing technical standards, as well as guidelines and recommendations. The article also identifies and highlights a set of problematic issues that arise, threatening to jeopardise the legitimacy and credibility of their rule-making.