Flexor hallucis longus (FHL) muscle has several functions, including plantarflexion of the ankle, flexion of the big toe and support of the medial longitudinal arch. To date, only a few studies have used non-invasive methods to examine FHL behavior during real-life movements, although the proper functioning of this muscle may be important to increase sport performance, the altered FHL function may contribute to the susceptibility to Achilles tendinopathy and the function of FHL is altered in other clinical conditions such as flatfoot. FHL seems to be multifunctional but other active and passive structures also contribute to the same functions. Because of the multifunctionality of the foot structures and FHL itself, it is hard to define the main role of FHL during certain kinds of tasks and real-life movements. It is also more difficult to determine how morphological changes in the FHL affect its function. In this review we summarise studies that have examined FHL function focusing particularly on functionally relevant in vivo measurements, and possible effects of FHL anatomical and functional changes. Furthermore we make suggestions regarding studies that are needed in the future to fill the gaps in our knowledge about the role of FHL.