Objective: This research explores the role of social capital in urban citizens' initiatives in the Netherlands, by using in-depth interviews. Method: Social capital was operationalized as shared norms and values, connectedness, trust and reciprocity. Results: The findings show that initiatives form around a shared idea or ambition (shared norms and values). An existing network of relationships (connectedness) is needed for an idea to emerge and take form. Connectedness can also increase as a result of an initiative. Some level of trust between people needs to be present from the start of the initiative. For the initiative to persist, strong in-group connections seem important, as well as a good balance between investments and returns. This reciprocity is mainly about intangible assets, such as energy and friendship. Conclusion: This study concludes that social capital within citizens' initiatives is both a prerequisite for the formation of initiatives and a result of the existence of initiatives.