Outsourcing in the logistics sector is an omnipresent issue. Transport service is especially affected by this trend because it is often not ranked among the core competences of the company and therefore often one of the first services externally assigned. The cooperation with transport service providers requires a performance measurement system to monitor the performance and the quality delivered. The focus of this research is on identifying the optimal parameters for measuring the performance of the transport service provider engaged. To clarify this central issue, a literature research as well as an empirical study in form of three expert discussions have been conducted. In order to be classified as an expert, a person needs to be employed at a logistics company which cooperates with or is a transport service provider and needs to be responsible for the coordination and measurement of the transport duties. Those findings obtained from the literature research and the empirical study have been merged to acquire the general conditions for an optimal system of indicators. The aim of this analysis was to develop recommendations for actions as well as a system of indicators which can be used as a general instrument for measuring the performance of transport service providers. As a result four important indicators have been identified. These can be used for measuring the quality of the external transport service providers.