This diploma thesis called "Influencers in Social media and their Use in Marketing" deals with the new current trend in which, instead of well-known celebrities, star just ordinary people. Despite of this, they managed to conquer the masses through social networks thanks to their unique personality and authenticity. The first chapter focuses on online marketing and explains the basic concept of it, its tools and communication mix, along with data on the structure of the Czech Internet population. The second chapter is devoted to the social media and it also explains Word-of-mouth marketing, as it is one of the ways in which awareness of influences is spread. The third chapter aims to familiarize the reader with the YouTube portal and it also includes statistics about the Czech YouTube. The last chaptermedia. The aim of this master thesis is to give a comprehensive overview of the phenomenon of influencers, with particular regard to YouTubers who represent the influencers of the present. The practical part of this thesis is the to identify target group of influencers (YouTubers) and the subsequent characteristic of this target group.