Authenticity has been theorised in a variety of overlapping ways in previous literature. In this thesis theories of authenticity are synthesised and categorised overall as theories of personal, nominal (material) and expressive authenticity, and processes of experiencing, judging, negotiating and presenting authenticity. Consumption-based authenticity has been theorised as experienced in individual activities and negotiated in social contexts. At the same time the presentation of authenticity is of growing interest in the context of online user-generated content (UGC). UGC on blogs and YouTube has grown exponentially in recent years, attracting significant audiences and consequently attracting managerial attention. Practice theories provide an appropriate approach to researching these combined consumption and media practices, and how they involve the presentation of authenticity. Here a practice-based approach is used to explore the UGC practices of beauty bloggers and YouTubers from the Republic of Ireland, as experiences of authenticity, and with respect to how authenticity is judged, negotiated and presented. 23 interviews were conducted with 25 bloggers/YouTubers, and netnographic research conducted over a 24 month period via observation of beauty blogs and YouTuber channels. Online data from 4,916 blog posts and 1,844 YouTube videos was reviewed and analysed. This study contributes to theorising authenticity in general, and to extending theories of consumption-based authenticity and the presentation of authenticity online. The empirical base is extended via an understanding of blogger and YouTuber practices at a micro-level, highlighting the importance of testing products, on the content creator’s own body, as material authenticity, and the processes of developing skills and competences. Social connection is identified by, and among, the bloggers and YouTubers in the study, both online and off, and authenticity is negotiated within this social context via ongoing trade offs in relation to self-presentation, brand relationships, product review and product testing. Practices are key to identity formation and here the bloggers and YouTubers in this study negotiate hybrid identities.