To aid understanding of physics related to fast ions in the Large Helical Device (LHD) with a three-dimensional shape, a comprehensive set of fast-particle diagnostics has been developed. Fast ions (H+) have been created by neutral beam injection and ion cyclotron resonance heating in LHD discharges. Intense fast-ion populations not only heat the bulk plasma but also drive collective instabilities under certain experimental conditions. Fast-ion experiments on LHD have been conducted to investigate neoclassical confinement of fast ions and issues on anomalous transport of fast ions induced by fast-ion-driven instabilities. This paper reviews fast-particle diagnostics that are essential for study of fast-ion physics on LHD.