Cambridge Assessment has more than 20 years experience in assessing Critical Thinking (CT) in a number of diverse tests and qualifications, unrivalled by any other body within the UK. In recent years, a number of research activities have been carried out in order to support these assessments, with a focus on the validity of measurement. This paper will describe the programme of work, related to four areas of research: (1) to derive a definition, taxonomy and glossary of CT and the importance of such work not just for test design but also for syllabus design and question setting; (2) to survey the practical issues surrounding teaching CT in schools, the experiences and perceptions of teachers; (3) to examine the impact of taking A/AS levels in CT on students' performance in their other A-level subjects; and (4) to examine the predictive validity of HE admissions tests containing CT on subsequent university exams and degree performance. Finally, the paper will discuss how these research activities contribute to the validity of CT tests and qualifications. (Contains 4 figures and 4 tables.)