This article reviews the development of piezoelectric energy harvesters (PEHs) that utilize water flow as a power source over the past 25 years. The review covers various aspects, including excitation mechanisms, oscillation structures, piezoelectric materials, power management interfaces, and applications. The energy-flow theory is used to discuss the three stages of energy extraction, conversion, and transfer. The review highlights the advantages and disadvantages of different excitation mechanisms, such as vortex-induced vibration (VIV), galloping, wake-induced vibration (WIV), turbulence-induced vibration (TIV), cavity-flow-induced vibration (CIV), blocking, and wave motion. It also discusses the oscillation structures, including cantilever beams, flapping eels, and compression structures. The performance of PEHs is evaluated based on parameters like effective flow velocity, oscillation frequency, and amplitude. The working modes of piezoelectric materials, such as d33, d31, and d15, are explained, and the most commonly used materials, lead zirconate titanate (PZT) ceramics, macrofiber composites (MFCs), and polyvinylidene fluoride (PVDF) polymers, are reviewed. The review aims to provide a comprehensive understanding of water-flow PEHs and guide future research and development in this field.This article reviews the development of piezoelectric energy harvesters (PEHs) that utilize water flow as a power source over the past 25 years. The review covers various aspects, including excitation mechanisms, oscillation structures, piezoelectric materials, power management interfaces, and applications. The energy-flow theory is used to discuss the three stages of energy extraction, conversion, and transfer. The review highlights the advantages and disadvantages of different excitation mechanisms, such as vortex-induced vibration (VIV), galloping, wake-induced vibration (WIV), turbulence-induced vibration (TIV), cavity-flow-induced vibration (CIV), blocking, and wave motion. It also discusses the oscillation structures, including cantilever beams, flapping eels, and compression structures. The performance of PEHs is evaluated based on parameters like effective flow velocity, oscillation frequency, and amplitude. The working modes of piezoelectric materials, such as d33, d31, and d15, are explained, and the most commonly used materials, lead zirconate titanate (PZT) ceramics, macrofiber composites (MFCs), and polyvinylidene fluoride (PVDF) polymers, are reviewed. The review aims to provide a comprehensive understanding of water-flow PEHs and guide future research and development in this field.