In recent years, customer retention has gained increased value among both goods and service providing firms. However although extensive research exist on the concept of customer retention and its measures and instruments, studies and research on how professional service firms retain their customers remain limited. Hence, in this thesis how professional service providers retain their targeted customers over time will be investigated through four case studies in the professional service industry, more specifically in the advertising sector. The empirical data was collected through interviews with four Swedish advertising agencies: Lowe Brindfors, Favör Reklambyrå, Holy Diver and Vinter Reklambyrå. The findings of this study first and foremost indicate that professional service providers do not have any formal nor standardized procedure which they follow when it comes to retaining their customers. The strategies employed by the firms are highly customized to each individual customer. Further the findings from this study indicate that in order to retain customers over time professional service providers need to place more efforts on the creation of personal relationships with the clients, as it is a strong bond tying customers to the firm. The findings further imply that the creation of customer satisfaction and the creation of switching barriers are the main strategies employed by firms, for retaining customers. Other factors affecting professional service firms’ retention strategies are the firms’ ability to convey confidence, to get the customers involved, and to be able to deliver good quality services.