Vascular malformations are rare congenital tangles of dysfunctional and dilated vessels. The lesions vary in size, can occur anywhere in the body and are often visible as a stain or three-dimensional mass differing in colour and texture from normal tissue. Common symptoms are pain, disfigurement, impaired physical functioning and psychosocial problems. In this thesis, steps were made towards harmonising outcome measures in the clinical research on peripheral vascular malformations. This thesis highlights the need for the use of uniform outcome measures in the evaluation of treatment effects on vascular malformations and the use of patient-reported outcome measures (PROMs). In a worldwide collaborative effort between patients and clinical experts, we determined what needs to be measured when evaluating treatment effects. Existing PROMs were evaluated and selected for measuring generic core outcome domains related to quality of life. For the measurement of condition-specific core outcome domains, no adequate instruments were identified, after which a new condition-specific PROM was developed and presented in this thesis. With the completion of this thesis, outcome measures for evaluating treatment effect in patients with vascular malformations have been determined. This thesis provided valuable insight into what and how to measure when evaluating patient-centred treatment effects for patients with vascular malformations. Our findings are expected to allow for the determination of disease severity and uniform assessment of the effects of treatments, which has been lacking up till now. The work presented in this thesis enables us to fill in many essential knowledge gaps in the field of vascular malformations, with the ultimate goal to improve the care for patients with vascular malformations.