Inter-organizational collaboration plays a pivotal role in the provision of care for patients and clients. Yet, questions on how and why healthcare providers collaborate, how collaboration coexists with competition policy and how collaboration contributes to patient outcomes were largely unexplored. Therefore, this thesis studies inter-organizational collaboration between providers within the setting of regulated competition. The results show that objectives for collaboration differ between hospitals and long-term care organizations. Competition enforcement aims to safeguard competition when collaborators are also competitors. The findings of this dissertation reveal a distinct shift in enforcement approach, from punitive enforcement to providing informal guidance. Collaboration in oncological care often aims to meet minimum volume standards or increase volume of care. Analysis breast cancer surgery outcomes shows that most variation is explained by differences in treatment types as well as patient and tumor characteristics, while hospital volume and regional competition seem to play a limited role. A preliminary evaluation of a centralization agreement for high-complex tumors does not demonstrate clear differences in price and travel time.