The view to complex interconnections between different companies in the supply chain shows that almost every industrial company relies on the external procurement of goods and business services. The external acquisition of business services has received much attention in recent years. Depending on the industry and the field of business activity, externally procured business services account for up to 80% of a company's entire purchasing volume. Contrary to this development, however, buying companies face various difficulties during the procurement and evaluation of business services. In addition to the pronounced heterogeneity of business services, their intangible characteristics also prevent a uniform assessment based on clearly quantifiable criteria, known from the purchasing of goods. The current thesis focuses on the topic of service procurement and examines different determining factors that influence the purchasing and evaluation of business services. More precisely, these factors include the heterogeneity of business services, the integration of the purchaser into different purchasing phases, and the internal collaboration with both internal customer and the external service providers. The theoretical basis lies upon agency theory, service-dominated logic, contingency theory, and industrial marketing and purchasing theory. Methodologically, this research builds upon two explorative case studies and a confirmatory analysis that uses structural equation modelling with 208 participants from GAS countries. The research results reveal that the service type has a significant impact on the procurement and evaluation of business services. Depending on the strategic relevance of the acquired business service, a different exchange takes place between all involved entities. Following this argumentation, the service type further influences the extent to which the purchaser's integration during different purchasing phases supports col.