This overview provides a fundamental understanding of citizen engagement (CE) and its relevance to the evidence-informed policy (EIP) work of the World Health Organization (WHO) and its Member States. The document introduces readers to the rationale and concept of CE, outlining its conceptual strengths, implications and practical applications. It serves to justify and promote the integration of citizens’ voices as a crucial and underutilized form of evidence in policy- and decision-making. This overview document is the first in a series of WHO publications on the topic of CE in EIP. Subsequent resources will include practical guides and toolkits.