This dissertation examines how the balance between security and liberty developed within the United Nations and European Union counterterrorism sanctions between 1999-2020, viewing the security-liberty relationship as a dynamic, time-dependent process rather than a static trade-off. It investigates whether and how the UN and EU moved towards a shared understanding of safeguarding human rights while addressing terrorism, introducing an updated security-liberty balancing framework with a time qualifier, combined with norms convergence theory, to offer a nuanced account of their interaction on counterterrorism sanctions and human rights protection. The study argues that an extended timeframe is required to demonstrate that security and liberty norms continually evolve, and that traditional approaches to the balance therefore need revision. While the time qualifier reflects this ongoing evolution of security and liberty, norms convergence theory illustrates how this relationship shifted over the past two decades. By analysing four convergence pathways, politico-strategic, legal, procedural, and organisational, the researcher demonstrates that a variety of motivations, goals, and actions related to UN and EU counterterrorism sanctions must be considered separately. Norms convergence theory highlights the diverse motivations, goals, and actions of UN and EU actors and norms entrepreneurs involved in counterterrorism. Methodologically, document analysis provides the primary evidence base, combined with semi-structured expert interviews, and complemented by selective use of process tracing, event history and network analysis to identify when, why, and how significant shifts in sanctions regimes occurred. The findings indicate that major terrorist attacks, legal challenges, and institutional reforms repeatedly prompted both organisations to adjust their sanctions regimes and gradually incorporate clearer safeguards, including listing criteria, review mechanisms, and exemptions. Over time, these developments produced a closer, though still incomplete, alignment between UN and EU norms on security and liberty, driven by legal accountability, institutional learning, and multilevel norm entrepreneurship rather than by overarching counterterrorism strategies.