Visual arts theory is fundamental to facilitating visual literacy, or students' ability to decode and construct imagery. Visual literacy skills support students' participation in contemporary society. This doctoral study uses a mixed methods approach to investigate students' engagement in visual arts theory, as increased engagement may facilitate visual literacy skills. A diagnostic instrument was created to measure year 11 students' prior learning in theory, as well as their cognitive and psychological engagement. Interviews with year 11 students, visual arts teachers, and some principals or school representatives, facilitated the development of the instrument and contextualised the findings. Phase One findings suggest measuring students' engagement facilitates the diagnosis of key issues and knowledge gaps affecting students' engagement in visual arts learning.