Addressing environmental issues is becoming increasingly important for businesses and NGOs. In addition, through the internationalisation of these organisations, the problem is becoming global. To address environmental issues in an effective way, there is a need to form a cross-sector collaboration between NGOs and businesses. Thus, to have a successful collaboration, it is important to search for suitable partners. This thesis investigates the partner search process of NGOs for cross-sector collaboration. It investigates how the process of partner search with regard to drivers for collaboration, formation typology, success factors compatibility, resources, and capability, as well as trust affect the partner search of three NGOs with different internationalisation degrees. A literature review has been conducted to investigate what has been discussed regarding the key concepts: environment, internationalisation degree, partner search, formation, drivers, and success factors. The resulted conceptual framework is used to analyse the empirical data. The study was conducted following an abductive approach since existing literature is only existent on firm-perspective so far. To investigate the NGO´s partner search for potential business partners, the thesis follows qualitative research where semi-structured interviews were conducted to create three single cases. The cases represent three internationalisation degrees, namely local NGO, regional NGO, and international NGO. The analysis discusses the differences and similarities between the empirical findings and the theoretical findings. The conclusion suggests that the internationalisation degree influences the partner search process slightly in every key concept. This is mainly due to the interdependence between each concept, which challenges a clear distinction. Nevertheless, the main implication for theory is the contribution to the so-far missing NGO perspective on the partner search in cross-sector collaboration. This thesis also provides practical implications where it appeals for NGOs to clearly understand the intention of the business partner for the collaboration and build trust. The study concludes with limitations and future research implications.