With the growth of modern systems, there is an urgent need to comprehend the epistemic activities of engineers in the design, operation, maintenance, and decommissioning of systems. Typically, these epistemic activities are characterized with the label of technological or engineering knowledge. However, in this article we argue that in complex systems, there is a need for engineering understanding along with engineering knowledge. This paper develops the notion of engineering understanding in relation to existing literature in philosophy of technology as well as philosophy of science and epistemology. It highlights insights from engineers interested in philosophy, as well as from philosophers interested in science and technology, and considers why engineering understanding is vital as a concept and needs further elaboration in the academic literature. We illustrate our claim in the example of troubleshooting problems in transportation systems that levy toll taxes, thereby demonstrating that problems of engineering cannot be handled by discussions of knowledge alone. Hopefully, this engagement with engineering understanding will also help the engineering community in self-reflection and clarification of the epistemic activities of engineers.