The 'responsibility to protect' (RtoP) concept has emerged rapidly over the last decade to take a prominent place in international discussions about the protection of populations from mass atrocities. However, little attention has been paid to the meaning of the term 'responsibility' in RtoP. 'Responsibility' is a slippery term that can perform a range of functions. This article suggests that, in order to understand the meaning of 'responsibility' in RtoP, we need to examine three things: what actors are responsible for; who actors are responsible to; and the ways in which irresponsible actors may be held to account. Such examination enables us to comprehend better some of the key debates and dilemmas with which the RtoP principle continues to be confronted.