Critical and Creative Thinking (CCT) is a core 21st Century competency. Is CCT ability a single, coherent construct that can be accurately measured, however? The Australian Curriculum, Assessment and Reporting Authority (ACARA) conceives CCT as a general capability that consists of four interrelated elements: Inquiring, Analysing, Generating and Reflecting. A study was carried out that tested whether CCT as defined by ACARA is a single construct that can be applied across school levels from Year 1 to Year 10. Trial data consisted of responses from 4,954 students in 48 Victorian Primary and Secondary schools to (various subsets of) 312 assessment items. Using Rasch measurement theory it was found that most items fit a single uni-dimensional model quite well. In this paper I outline the CCT construct, explain how the assessment instruments were developed and discuss characteristics of the final CCT scale.