Abstract : In this thesis, we analyze the contractual relationship between private equity investors and entrepreneurs/managers of small and medium sized firms. More specifically, we focus on the real contribution of these financial intermediaries to the Initial Public Offering (IPO) performance of financed firms. While considering venture capitalists' involvement in the governance of financed firms as the main explaining factor, we use the Corporate Governance theory as the relevant theoretical framework. Thus, we firstly point out the agency theory perspective and secondly the resource and knowledge based view perspective to examine the determinants of venture capitalists' involvement and the impact of this active participation of private equity investors to financed firms' performance. From the agency theory perspective, venture capitalists appear as active investors taking part in the governance of finance firms in order to reduce agency costs that may arise from the conflicts of interest existing between both parties. The governance structure aims at monitoring and inciting entrepreneurs to work in the company's interests. In a resource and knowledge based view theories, these investors add more value to firms trough generating some additional knowledge, skills and facilitating knowledge transfer among parties. The entire creation process is therefore analyzed and the governance mechanisms appear as channels sustaining knowledge generation and transfer. In this sense, venture capitalists' characteristics, such as their expertise and experience are more than critical to the success of IPOs. These two perspectives of venture capitalists' contribution are tested from an empirical point of view through a sample gathering more than 500 IPOs occurring on the Financial French market in this period 1997- 2005. Our findings reveal that the monitoring role of venture capitalists through governance mechanisms is amplified by the level of information asymmetry and the degree of conflicting interests existing between both parties. While considering the resource and knowledge based views, it appears that venture capitalists may have a greater contribution depending on their expertise, experience, knowledge of the financed sector... In addition to these results, we find that the IPO performance of venture backed firms is correlated to the governance structure set up by these investors. The number of syndicated investors, the heterogeneity of skills between co-investors and the diversity of skills among board representatives appear as relevant governance factors related to the IPO performance of venture backed firms. Thus, this study contributes to the venture capital literature from a theoretical and empirical point of view. It also has some implications for the manager of small and medium sized firms.