Intermodal road, rail, air and sea transport networks across the European Union (EU) calledTrans-European Networks for Transport (TEN-T) are currently being implemented. Keyelements, which should be implemented by 2030, are the most strategic TEN-T Core networkcorridors. These nine corridors are expected to enhance smoothness and sustainability oftransport across the EU, thereby enhancing cohesion between EU nations, and relate to non-EU nations. In the EU Regulation 1315/2013 a governance framework for implementation ofthe Core network corridors is outlined, which mainly involves appointment of a Europeancoordinator for each corridor, establishment of Corridor forums for consultations, andformulation of work plans and follow-up measures.This framework is now being implemented in practice. A recognised challenge in the processis engaging the numerous, diverse stakeholders. A possible solution is to introduceappropriate elements of complementary governance. Thus, in this doctoral research, suitablecomplementary governance to support the sustainable development of a Core networkcorridor was investigated. Complementary governance, a concept that was developed andrefined during the project, refers here to additional governance in relation to the governanceframework described for the EU Core network corridors in the EU Regulation 1315/2013.To address an identified gap in knowledge on the nature and utility of complementarygovernance a series of studies was carried out. The studies included a study of relevantacademic literature, studies of EU transport policy and implementation documents regardinggovernance and sustainability, a survey of views of Scandinavian-Mediterranean Corridorforum participants, and interviews with stakeholders in the Baltic Sea region. These studiescollectively constitute a case study, focusing on the Scandinavian-Mediterranean Corenetwork corridor, of the need for, and if a need is present the optimal design of,complementary governance in the process of sustainable development of a Core networkcorridor.The main findings of the studies are that the implementation process of the Core networkcorridors could benefit from complementary governance measures, mainly to increaseinclusiveness, which is crucial for a sustainable development. Complementary governancecould both deepen and broaden stakeholder awareness and involvement. This may haveseveral advantages, such as increasing possibilities for stakeholders to influence the processand benefit from the corridor, align activities, and promote cooperation and learning betweenstakeholders. Moreover, including stakeholders who are based outside the corridors mayimprove distributive aspects of the investments and reduce risks of focusing attention tooclosely on the corridors per se. However, broadening stakeholder inclusion also hasdisadvantages, as it increases the complexity of the process, which may impair itseffectiveness, and may lead to unrepresentative involvement due to differences instakeholders’ interests, administrative capacities and power. It also increases requirements forcoordination.The nature of complementary governance is multifaceted and related to diverse dimensions ofgovernance, such as processes, stakeholder participation and implementation documents. For complementary governance measures a flexible and task-specific multi-level design isemphasised, which may involve various types of engagement mechanisms. Further, thefindings showed a need to extend previously recognised types of public engagementmechanisms based on communication, consultation and participation in the decision-makingstage, by recognising another category, named stakeholder exchange. This category ischaracterised by stakeholder cooperation and collaboration in an implementation stage wheremain goals and policies already are in place.Areas for complementary governance in the implementation process of the Core networkcorridors were indicated in the interview study. They consisted of extended involvement ofprivate stakeholders, extended involvement of regional and local stakeholders, extendedinvolvement of stakeholders geographically external to the immediate corridor, and extendedinformation both generally and specifically (i.e. directed information to specific stakeholders).Several complementary governance measures that could potentially improve implementationof the Core network corridors were raised in the interviews, ranging from minor adjustment inthe existing process to entirely new solutions. These ideas correspond well to the indicatedareas and provide robust foundations for further discussion.Thus, findings from the research suggest several governance measures to systematicallyinvolve and engage diverse stakeholders in the Core network corridor implementationprocess. However, before introducing such measures, they should be assessed together withexisting governance measures, to avoid overlapping or contradictory initiatives, possiblyusing policy packaging approaches, where policies are combined and analysed together toachieve a desired overall outcome. Proposals for future research and practices are presented.