The phenomenon of sustainability is one of the hottest topics in today's society because ofthe ongoing climate crisis that threatens the well-being of the planet. The fast fashionindustry is one of the world's most polluting industries that is continuously faced withchallenges when it comes to implementing sustainability in its operations. The main purposeof this industry is to produce large quantities of clothing at low prices with a short life cycleto be able to regularly launch new fashion campaigns, which triggers impulse buyingbehaviour among consumers. In order to protect the planet, it is important that there is achange in consumer attitudes, while demands must be placed on fast fashion companies thatcan make them integrate sustainability into their business models. According to previousresearch, there is a weak correlation between sustainable consumers' attitudes and theiractual purchasing behaviour regarding apparel consumption, which leads to the creation ofan attitude-behaviour gap. The fundamental purpose of this study is to find out how internal and external circumstancesinfluence sustainable consumers' buying behaviour and make them turn to fast fashioncompanies despite the positive attitude they initially had. This will contribute to increasedknowledge and understanding of the attitude-behaviour gap and the circumstances that leadto the emergence of this phenomenon. Thus, the study intends to answer the followingproblem formulation: What circumstances influence conscious consumers to purchase fast fashion? The literature review of in the study is based on previous research in the areas of attitude-behaviour gap, consumer decision making process, theory of planned behaviour, externaland internal circumstances. In order to be able to answer the research question, the thesisauthors have chosen to start from a qualitative research method. In the study, a total of 8female respondents between the ages of 24-28 participated and these respondents have beenselected using a so-called snowball sample, in order to be able to include individuals with asustainable mindset, which is relevant for this study. In order for the interview participantsto feel comfortable talking openly about the topic, thesis authors have therefore avoided toinclude family and friends in the interviews. The authors have instead used their contactnetworks to get in touch with relevant respondents to this study. The results of the study show that external and internal circumstances- compromisingproduct attributes, limiting conditions and perceived risk, collectively contribute to thecreation of an attitude-behaviour gap and cause sustainable consumers to choose to turn tofast fashion companies when buying clothes.