Fibroblast growth factor homologous factors (FHFs) bear strong sequence and structural similarity to fibroblast growth factors (FGFs). However, the biochemical and functional properties of FHFs are largely, if not totally, unrelated to those of FGFs. Whereas FGFs function through binding to the extracellular domains of FGF receptors (FGFRs), FHFs bind to intracellular domains of voltage-gated sodium channels (VGSCs) and to a neuronal MAP kinase scaffold protein, islet-brain-2 (IB2). These findings demonstrate the remarkable functional adaptability during evolution of the FGF gene family. FHF gene mutations in mice result in a range of neurological abnormalities, and at least one of these anomalies, cerebellar ataxia, is linked to FHF mutations in humans. This article reviews the sequences and structure of FHFs, along with our still limited understanding of FHF function.