Health Technology Assessment (HTA) requires several disease-level inputs including a natural history model, quality of life and cost of illness evidence. In rare diseases there may be little published evidence, few clinical experts, and a limited pool of patients to develop these inputs. A patient-led collaborative approach to building tools and evidence for HTA can be effective and deliver high quality inputs to HTA that reflect patients’ perspectives and experiences. Duchenne UK’s Project HERCULES has delivered a disease level economic model, natural history model, a validated patient-reported outcome measure for health-related quality of life in multiple languages and projects to review healthcare resource utilisation and the cost of illness. This chapter describes how patient input was obtained alongside that of clinicians, technology developers, and decision makers to steer rigorous academic work. This was achieved through regular engagement with all stakeholders throughout the project, using patient expert input to identify where current methods were insufficient, set the scope of new work, provide input on emerging new methodological approaches, feedback on emerging results and and issues, and shape outputs.