International organisations (IOs) wield considerable influence in today’s world. Distinguishing them from other actors are the new ideas they produce and communicate that can reframe global debates. However, there is little research about how these organisations evaluate their communication activities. This thesis sets out to fill this gap by providing the first in-depth study of communication evaluation within IOs. The central question of this thesis is to assess the extent to which communication evaluation is possible within IOs with three specific questions: 1) the appropriateness and feasibility of communication evaluation methodology for IOs; 2) the influence of internal and external factors; and 3) the use of communication evaluation findings in IOs. These questions are addressed in three distinct but interlinked empirical studies framed by a conceptual framework. Article 1 provides a 15 year review of communication evaluation within IOs through analysing systematically evaluation reports. Article 2 provides an analysis and reflection on the evaluation by this author of two communication campaigns of IOs. Article 3 provides a comprehensive study on the use of evaluation findings of these two communication campaigns. This thesis found that a process of conceptualisation is needed to match the given communication activities to an appropriate evaluation methodology, implemented with a pragmatic, adaptive and participatory approach rather than imposing a standard set of methods. Evaluation was found only in a minority of IOs and there are challenges in matching evaluation methods to the range and complexity of their communication activities. Internal factors of organisational context and communication goals were found to be important and to equally impede and enable the evaluation process. Improving the efficiency of communication activities was the main use seen of evaluation findings, occurring in unexpected and often opportunistic ways, but nevertheless an indication of a specific contribution of evaluation for communication professionals.