There is an apparent lack of understanding of the tidal resource and its response to harvesting of energy among the principal proponents of the fledgling UK tidal energy industry: the device developers, principally supported by government funding. The authors' intent is to widen understanding of the different hydrodynamic conditions and controls that generate favourable conditions for tidal energy extraction. This paper outlines the generic hydrodynamic conditions that typically provide the most important fundamental requirement for harvesting tidal energy, namely, a large tidal current resource. Five generic tidal regimes are presented, and the suitability of each regime for harvesting of energy is considered. Of the five regimes, two cases are identified as being most prevalent, but are generally unsuitable for economic exploitation. Understanding the significant differences between the driving mechanisms of each of the flow regimes is therefore key to effective site selection for large-scale development. Furthermore, the response of the different regimes to energy extraction from the system is not consistent, further impacting on site selection. Numerical simulations of idealized examples highlighting the different hydrodynamic conditions in operation will be used to support descriptions of the relevant tidal flow regimes.