This thesis explores the influence of User-Generated Content (UGC) on the purchasingdecisions of millennials in the fast fashion industry, particularly through the platformsInstagram and TikTok. The research delves into the transformation from offline to onlinemarketing within the fast fashion sector, examining both fast-fashion and ultra-fast fashion tounderstand UGC's impact on consumer behavior, including environmental and ethicalconcerns. Employing a qualitative research methodology, this study focuses on thematic and contentanalysis to unravel the intricate ways in which UGC shapes millennials' purchasing decisions.The research provides an in-depth exploration of the subjective experiences, perceptions, andmotivations of this demographic, offering rich insights into their interaction with UGC.Additionally, the study integrates social influence theory and buying behavior theory tounderstand the dynamics of digital consumer engagement and the role of UGC incontemporary marketing strategies. Through interviews with Millennial consumers and analysis of their interactions with UGC,the research uncovers patterns and trends that highlight the significant role of UGC in digitalmarketing and consumer decision-making. The results suggest that while UGC significantlyinfluences Millennials, the impact varies across different aspects of the fast fashion sector.This research fills a void in current literature by providing a focused analysis of UGC'sinfluence across generational divides, thus contributing to a deeper understanding of digitalbehavior in the fast fashion industry.