Background: Globally, over 2.5 billion people are estimated to be in need of assistive products (AP), but coverage is low. These estimates are uncertain as data is inadequate and lacks comparability. A survey methodology to assess population AP need is crucial to plan services. Study Aim: To investigate the development of population-based survey assessment methods to measure AP need in low and middle-income countries (LMICs) in the functional domains of vision, hearing and mobility and present methodology for a draft AP need survey tool. Methods: This thesis advances AP assessment through six linked studies investigating assessment methodology of AP need. 1) Systematic review of AP need estimates and review of identified functional assessment methodologies. 2) Secondary analysis of clinical and self-report assessment approaches and need for glasses, hearing aids and wheelchairs in population-based surveys in Cameroon and India. 3) Population-based survey of vision and hearing impairments in The Gambia to estimate need for glasses and hearing aids and compare clinical and self-report assessment approaches. 4) Population-based survey of rapid assessment of musculoskeletal impairment in Syrian refugees in Turkey to estimate AP need. 5) Population-based self-reported rapid assessment of assistive technology survey in Guatemala to estimate AP need and access. 6) Secondary analysis of five datasets (Cameroon, Chile, India, The Gambia, Turkey) to explore Washington group questions as a screening tool for population-based functioning for AP need. Key findings: Functioning: The systematic review found heterogeneity in assessment and reporting of AP need, emphasising the need to standardise data collection. A range of functioning assessment methodologies exist, however there is a gap for a fit-forpurpose functional assessment tool for use in surveys to assess AP need in LMICs. Impairment: The surveys advanced vision and hearing impairment (Cameroon, India, Gambia) and musculoskeletal impairment (Turkey) measurement protocols for AP. Poor agreement was found between self-report and clinical impairment assessment of AP need; this provides rationale for the development of a hybrid assessment tool. Self-report: The Guatemala survey estimated self-reported AP access, but recommended a hybrid assessment approach. Washington Group questions had moderate sensitivity and specificity in estimating AP need. Conclusion: This thesis provides recommendations for the development of a population-based hybrid survey methodology to estimate AP need in the domains of vision, hearing and mobility and presents a draft AP need survey tool titled the “Functional Needs Assessment Tool (FNAT)”.