This chapter examines the concept of financial sustainability within Research Infrastructures (RIs), a critical yet often underexplored dimension of their long-term viability. Positioned as long-term, publicly funded, non-profit entities, RIs must balance scientific excellence with effective managerial and business-oriented practices. The chapter defines financial sustainability in the RI context, outlines the primary conditions necessary to achieve financial stability, and describes the role of robust financial planning in supporting informed strategic decision-making. A concrete example illustrates how financial planning tools can enhance RI management.