This chapter addresses the act of categorising music, with a primary focus on the concepts of genre, style, and idiolect as utilised in rock music research. Having presented brief definitions of these and several other related concepts, I explore the different ways in which one can understand the relationships between them. Many pieces of the literature posit hierarchical relationships between genre and style, and style and idiolect; however, I suggest that comparatively independent relationships between the concepts may yield fruitful analytical results, allowing one to understand in greater nuance the ways in which rock artists sit within broader musical contexts, in both style and genre terms. In the final section, I promote several areas of enquiry requiring further research, as pertaining to the historical dimensions of styles—namely, what is meant and implied when we add temporal qualifiers to existing style and genre labels.