In addition to formal welfare institutions, there are many informal initiatives in the social landscape that are engaged in social work. These informal actors offer help, care or support to their members and visitors, and/or fulfil a bridging function to formal welfare organisations. They fulfil this role in particularly for target groups that experience distance or barriers to the formal care and welfare system. Despite the important role they play, these actors are often still unknown territory for both formal care and welfare actors and policymakers. Based on research from Belgium and the Netherlands, this article sheds light on this hidden voluntary social work and its characteristics. We also examine hidden voluntary social work in relation to formal welfare organisations and the (local) government. Despite the potential added value of cooperation, it is not easy to build up a sustainable and equal cooperative relationship, or for the government to effectively support hidden voluntary social work. Safeguarding the uniqueness of informal players is a particular concern.