Formally describing and assessing the difficulty of learning and teaching material is important for quality assurance in university teaching, for aligning teaching and learning activities, and for easing communications among stakeholders such as teachers and students. This paper proposes a novel taxonomy to describe and quantify the difficulty levels of exam questions and exercises encountered in engineering-related contexts. This paper also describes the development and piloting processes of the new taxonomy. The proposed taxonomy consists of two dimensions which describe the difficulty in understanding/explaining and using/applying a content unit. The piloting phase included ten purposefully selected experts in the field of control engineering, external to the project, who tested the performance, utility, ease of use, and clarity of the new taxonomy. The results indicate that the users were able to provide consistent and coherent assessments of the difficulty levels of 15 selected exam questions. The paper further discusses suggestions for improvement voiced by the participants to promote an even more consistent and coherent assessment of engineering students' mastery of the subject. [For the full proceedings, see ED625421.]