This thesis is about the consumers and the fast fashion brands sustainability, the aim is to better understand how consumers perceive the fast fashion brands' efforts of sustainability. As part of their Bachelor Business Administration at the University West of Trollhättan, Emilie Dubuisson and Héloïse Lanciaux developed this thesis during the spring term 2020 for the degree project. This thesis aims to better understand the perception of the different consumers on the unsustainable practices of the fast fashion brands and their efforts of sustainability. Fast fashion business model is based on an instant feedback of consumers' wishes to adapt the offer. Thus, the fast fashion brands can be interested in our results in order to better understand the demand of the consumers and to adapt their offer. The opinion of the consumers constantly evolves and it can be interesting for the brands to have recent information in order to better understand the new fashion consumers' consumption habits and still attract them. In the theoretical frame of references part, we highlight the fast fashion brands' unsustainable practices used to produce their clothes quickly and at low prices. This part helps to better understand what the consumers could reproach to the fast fashion brands. Moreover, we enumerate the fast fashion brands' sustainability efforts in order to better understand what they do to improve themselves. Then, we did some researches on the consumers' perception of sustainability to understand what they expect to fast fashion brands in term of sustainability. In order to support the theory and to bring new knowledges on this subject we conducted a quantitative study based on a questionnaire. In the empirical data we present the results of our research in which 88 consumers shared their opinion. In this part, we arrived at the conclusion that our two hypotheses are true: The different types of consumers are concerned about the fast fashion brands' unsustainable practices and it influence their decision purchases and the different types of fashion consumers do not perceive the fast fashion brands' efforts of sustainability as real efforts are both true. Finally, we crossed the different data from literatures and the existing knowledges with the data resulted from our questionnaire in order to construct the analysis part. We arrived at the conclusion that the consumers do not perceive the sustainability of the fast fashion brands. The consumers think that the brands make efforts on their marketing but they do not decrease their environmental impact or resolve their ethical and economic issues.