The term 'filling in' was developed in the context of research into UK devolution in order to move beyond the literature on the 'hollowing out' of the national state. We critically unpack the concept of filling in and assess its value, identifying three main ambiguities. First, we emphasise the need to distinguish between what we term structural and relational forms of filling in, ensuring a clear analytical separation between structure and agency. Second, we examine the associated scalar politics of devolution, arguing that filling in and hollowing out are not scale-specific processes. Third, we distinguish between filling in as originally advanced, which is time-specific in relation to the process of devolution, and 'pure' forms of filling in which can be seen as time-neutral. In abstracting filling in from the context of devolution in this way, our purpose is to render it capable of wider application to the study of other processes of state restructuring.