This paper offers an approach to assess the quality of foresight knowledge deployed or generated during foresight exercises. Although 'foresight knowledge' is a special case, the idea of having a procedure, an approach or a method available for assessing the quality of knowledge is tempting: Foresight knowledge consists of various sources such as different scientific disciplines, normative visions on the future, planning and scenarios. It also receives input from different sectors of society, such as industry, academia and civil society. How can we then assess the quality of knowledge inputs from such diverse sources and its implication for foresight exercises? This paper is an attempt to address this subject matter. Foresight knowledge and quality assurance is illustrated on issues of sustainable development.