A tidal bore is perhaps the most spectacular tidal phenomenon that can be readily observed. When a large tide enters a shallow, funnel-shaped estuary with a gently sloping bottom, its waveform is distorted and this can lead to an impressive rolling ‘wall of water’, travelling upriver. ‘Tidal bores’ explains that estuary shape and a large tidal range are important for tidal bore formation. Tidal bores can be smooth, non-breaking ‘undular’ waves or a variety of breaking forms of increasing violence. Famous examples are seen along the Qiantang River in China, the Amazon River in Brazil, and the River Severn in the UK. The impact of tidal bores on estuarine processes and ecosystems is also discussed.