The subject studied in the doctoral dissertation is the positive influence of factors, such as trust in the supplier, commitment of the supplier to the relationship with the retailer, non coercive power of the supplier and retailer dependence on the supplier, on development of long-term relationships with suppliers, as well as the possibility of gaining competitive advantage through long-term relationships with suppliers. In the process of internationalization and globalization and present high volatility of the retail sector, successful retailers find it difficult to maintain a competitive advantage. They recognize the development of long-term relationships with their suppliers as an opportunity to achieve a unique position in the sector and create an competitive advantage. Long-term relationships with suppliers develop over a long period of time and therefore cannot be easily taken over by competitors. To have a competitive advantage for a retailer means achieving a more favorable position than its competitors on the retail market. Achieving and maintaining competitive advantage is considered one of the fundamental goals of a retailer's business. Competitive advantage can have its sources in the company or it can be the result of unique relationships with the environment. Market changes demand a more aggressive approach in acquiring everything the company needs for achieving competitiveness. High volatility in retail is manifested through rapid fluctuations in customer demand and unpredictable market trends. Driven by such an intense competitive environment, retailers are increasingly relying on their supply chain as a source of competitive advantage. Therefore procurement and supply chain management, which includes cooperation with suppliers, is gaining importance within the company. Facing the volatility and diversity of the market, retailers are forced to develop relatively flexible relationships with partners in the supply chain to ensure an efficient response to unexpected market demands but also reduce dependence on suppliers. Suppliers occupy a strategic role in organizations and significantly contribute to the creation of competitive advantage and their activities positively affect the organization's operations. The relationship between retailers and suppliers in the supply chain is one of the most important elements of supply chain integration. Establishing successful relationships and managing them in each segment of the supply chain becomes a prerequisite for business success. Therefore, retailers enter into cooperative relationships with suppliers in order to achieve individual as well as common goals that lead to better business results and to competitive advantage on the market. In research of cooperation between retailers and suppliers conducted till now, it has been concluded that the relationship with suppliers, and its long-term nature, is important for achieving a competitive advantage for the retailer and that certain factors, such as trust, commitment, dependence and non coercive power, influence the success of the retailer-supplier relationship, the retailer's business results and competitive advantage of the retailer. The connection between the factors and their interdependence was also established. Although the positive effects of supplier relationship management are recognized, a large number of retailers have difficulties in initiating, developing and maintaining relationships with their suppliers. Following the analysis of the theoretical framework on relationship factors, cooperation between retailers and suppliers and competitive advantage in the supply chain, empirical research was conducted. The researched population consisted of retailers in the Republic of Croatia. Empirical research has determined a statistically significant positive correlation between the factors retailer's trust in the supplier, supplier's commitment to the relationship and supplier's power that is not based on coercion, with the development of long-term relationships with suppliers, as well as a statistically significant positive correlation between the development of long-term relationships with suppliers and the achievement of the retailer's competitive advantage. The research did not establish a statistically significant positive association between the retailer's dependence on the supplier and the development of long-term relationships. The research examined the differences among retailers with regard to the size of the retailer, origin of capital and market strategy of the retailer. Based on the results large retailers show a positive and higher correlation between trust, commitment and non coercive power and long term relationships with suppliers compared to small and medium sized retailers. Only for the variable dependence of retailer , the correlation was not established. Regarding the to the origin of capital the research determined that there are only statistically significant differences in the variables supplier commitment and long-term relationships with suppliers between retailers and therefore it was concluded that there is a moderating influence of the origin of capital on supplier commitment and long-term relationships with suppliers. International retailer have statistically significantly higher results on the mentioned variables. Differences between retailers with regard to the market strategy exist among retailers who implement a strategy of maintaining their position on the market and minor growth, but the differences are statistically low. Retailers with an aggressive growth strategy show higher results in comparison with retailers who moderate growth strategy.