There are challenges in establishing agreed definitions for some concepts, such as `art' or `social justice', as virtually everyone who approaches these terms has an agenda framed by their values. Herein lies the problem; global citizenship itself is a contested concept that contains other contested concepts, creating an internally unstable, mutable term that seems a shaky foundation upon which to build a framework for assessing student learning outcomes. In this chapter we seek to explore how these difficulties can be overcome in order to enable assessment of global citizenship education. First, we describe global citizenship and its importance in education, followed by a discussion of the rationale for assessing global citizenship, the kinds of measures and tools that have been developed, and what we can learn from them. We conclude that the rationale for designing assessment for global citizenship education has been firmly established; however, innovative teaching and learning calls for innovative assessments which conflate learning dimensions to form the complex items required to assess global citizenship knowledge, skills, attitudes and values. [Author abstract]