Although subnational entities play an important role in governance for sustainable development, they are often not recognized as decision-making actors in multilateral bodies, where an important part of the policy debate takes place. Adopting a Multi-Level Governance perspective, this article presents four alternative routes they can use to be involved in multilateral decision-making. It further zooms in on inter-subnational networks, an application of one particular route, called the direct route. Inter-subnational networks are associations between subnational entities based upon common interests. They have both external and internal objectives. On the one hand, they want to represent their members at multilateral organizations and influence decisionmaking. On the other hand, they are aimed at fostering cooperation between their members and at stimulating policy learning. This article focuses on the participation of Flanders in two networks in the area of sustainable development: nrg4SD and ENCORE. Flanders is an interesting case because of its exceptional degree of autonomy. The analysis concludes that Flanders is mainly (but not exclusively) interested in the internal dimension of the networks. It further reveals a low political involvement, which seems due to the subject of sustainable development itself.