The paper provides a comprehensive assessment of the accountability capacity of the two main EU watchdog institutions. The overarching research question is: how powerful are these watchdog institutions in holding the EU executive to account? The paper presents an expanded analysis of the formal and organisational accountability powers of these EU watchdogs and how these are exercised, based on an indepth case study. We apply an assessment framework of accountability powers developed previously. Both watchdogs have comparable formal powers when it comes to accountability. Despite these similarities, the EO and the ECA differ in their organisational power and in the exercise of their powers. In the organizational dimension, there are differences regarding leadership. In the exercise of power dimension, the EO has increasingly developed standards for good practice with an emphasis on transparency and principles of good administrative behavior, while also using its networks to further improve complaint handling.