BACKGROUND:Heel lifts, placed inside footwear are recommended for the management of numerous musculoskeletal conditions. Despite the potential therapeutic benefit of heel lifts, the mechanism(s) by which they exert their effects is unclear. The aim of this systematic review was to synthesise reported findings and summarise the effects of heel lifts on lower limb biomechanics and muscle function. RESEARCH QUESTION:Do heel lifts affect lower limb biomechanics and muscle function during walking and running? METHODS:Electronic databases (MEDLINE, EMBASE, CINAHL, SPORTDiscus, AMED) were searched from inception to April 2018. Studies were included if they (i) included participants without a limb length discrepancy or neurological condition, (ii) evaluated the effect of bilateral heel lifts that were removable (attached to the participants' foot (barefoot) or inserted inside footwear) or an existing feature of a shoe, and (iii) assessed lower limb biomechanics or muscle function during walking or running in asymptomatic or symptomatic participants. RESULTS:A total of 23 studies (377 participants) were included. Study quality, assessed using a Modified Quality Index, ranged from 5 to 13 out of 15. A large number of biomechanical parameters were assessed, but few effects were statistically significant. The differences that were significant and had a large effect size are described below. In asymptomatic participants, heel lifts of 10 mm decreased duration of swing phase (standardised mean difference [SMD] = -1.3) and heel lifts of at least 5 cm decreased velocity (SMD = -0.93) during walking. In asymptomatic participants, heel lifts of 15 mm decreased maximum ankle dorsiflexion angle (SMD = -1.5) and heel lifts of 12 and 18 mm decreased gastrocnemius muscle tendon unit length (SMD = -0.96) during running. In participants with restricted ankle joint dorsiflexion, heel lifts of 6 and 9 mm increased medial gastrocnemius electromyography amplitude (SMD between 0.68 and 0.98) during walking. In participants with haemophilia, heel lifts of 9 mm increased ankle joint maximum range of motion (SMD = 1.6) during walking. SIGNIFICANCE:Heel lifts affect specific lower limb biomechanical and muscle function parameters during walking and running. The clinical relevance and potential therapeutic benefits of these effects needs further investigation.