This Zero-Dose Learning Hub toolkit provides practical guidance for designing, implementing, monitoring, and evaluating immunization programs that focus on identifying and reaching zero-dose (ZD) and under-immunized (UI) children using theory-based approaches. It supports government stakeholders, donors, and implementing partners to develop clear and evidence-informed measurement plans grounded in theories of change that map the causal pathways from activities to outcomes and long-term goals. The toolkit also highlights how theory-based approaches help explain how and why programs work, by making underlying assumptions explicit, examining contextual factors, and documenting causal linkages, to generate actionable insights for strengthening future programming. 

Designed as a plain-language resource, the toolkit walks users through developing, revisiting, and revising theories of change, identifying appropriate outcomes, and integrating evidence and learning throughout implementation. It provides methods to assess whether programs achieved their goals and why they did or did not work, promoting adaptive learning and timely adjustments in dynamic and complex environments. By guiding users to reflect on context, articulate risks, and co-create pathways with stakeholders, the toolkit supports more effective, responsive, and sustainable approaches to reaching ZD and UI children and improving routine immunization systems.