While parliaments have long been neglected actors in the analysis of security policy, a research literature on the subject is growing. Current research is focused primarily on how parliaments, relying on formal legal competences, can constrain governmental policies. However, this research needs expansion in three areas. First, informal sources of parliamentary influence on security policy deserve more systematic attention as the significance of parliaments often hinges on contextual factors and individual decision-makers. Second, we still lack a systematic understanding of the effects of parliamentary involvement on security policy. Finally, the role of parliaments for the politics of security is almost completely uncharted territory. When parliaments become involved in security policy, does it foster transparency and contribute to the politicisation of security policy so that security policy becomes a 'normal' political issue? The article reviews current research, derives findings from the contributions to this Special Issue, and spells out their wider implications.