Energetic tidal-stream environments are characterized by frequent, variable, yet broadly predictable currents containing ephemeral flow structures that change across multiple spatio-temporal scales. Marine mammals and seabirds (marine megafauna) often frequent such sites, but increasingly these locations are targeted for renewable energy extraction; little is known, however, about how marine megafauna use these habitats and any potential impacts. This review aims to summarize existing knowledge concerning usage by marine megafauna and considers their wider ecological significance. The review describes the physical processes occurring within tidal-stream environments that generate the oceanographic structures of potential ecological relevance, such as jets, boils, eddies, and fronts. Important physical features of these environments include lateral transport, turbulence-driven 3-dimensional flow structure at various spatial scales, and upwelling. Foraging opportunities appear to be the main attractor to marine megafauna, likely driven by enhanced prey abundance, vulnerability, or diversity. Many megafauna associate with particular tidal phases, current strengths, and flow structures, most likely in response to tidally forced prey distribution and behaviours. Occupancy patterns, distributions, and foraging behaviours are discussed. Local site fidelity by 'tidal-stream experts' suggest non-uniform conservation risks within larger metapopulations. The review discusses data-gathering techniques and associated challenges, the significance of scaling, and information gaps.