Background: Although many core sets of measurement concepts have been published in the literature, this has not been done for the field of lower limb orthoses. Objectives: This paper provides an overview of the measurement concepts that are relevant in lower limb orthotic evaluations, and it proposes a candidate Core Set of outcome measures to be used in clinical studies on ankle-foot orthoses (AFOs) and knee-ankle-foot orthoses (KAFOs). Study Design: Literature review. Methods: The International Classification of Functioning, Disability and Health (ICF) was used as framework to select relevant concepts. Results and conclusion: Measurement concepts covering all ICF levels of functioning were identified as relevant for the Core Set, including functions of the joints and bones (b710-b729), muscle functions (b730-b749), gait pattern functions (b770), walking (b450), moving around in different locations (d460), and daily-life functioning (d5-d9). Further validation of this candidate Core Set through a formal decision-making process is needed to obtain consensus among experts in the field. Based on such a consensus, the next step will be to systematically review the literature and identify those measurement instruments that are best suited to assess the proposed concepts, based on their psychometric properties in a given sample and context. Thereafter, we suggest that this ICF Core Set of measurement instruments should be applied in orthotic studies on AFOs and KAFOs in ambulatory patients with gait problems. Clinical relevance Although many ICF Core Sets have been published, this has not been done for the field of lower limb orthoses. We feel that such a Core Set is urgently needed, to enable comparison of results, and establish evidence on the efficacy of orthotic treatment, which will improve patient care. © The International Society for Prosthetics and Orthotics 2011.