This paper outlines some of the issues which need to be considered when analysing the extraction potential of a tidal current resource. Site selection is not a simple case of identifying an energetic site with an appropriately large peak tidal current. The characteristics of the current throughout the lunar tidal cycle must be considered. Furthermore, implicit in such an analysis is the assumption that the local tidal flow conditions will not be significantly altered by the energy extraction process itself. For high extraction rates, the general validity of this assumption is questionable. The influence of energy extraction upon the underlying hydraulic nature of the tidal environment must be considered. Analysis based upon open channel flow theory demonstrates that energy extraction in a simple channel driven by static head differences can have a significant upstream and downstream effect. This suggests that the environmental impact of energy extraction is not necessarily restricted to the immediate area around the extraction site. It also suggests that there is potential for the process of energy extraction to either diminish or even enhance the available resource at a particular site. Further research is required and is ongoing in this area. In the case examined, the limits to exploitation are shown to be inexact. However, a useful approximate guideline for resource analysis would be that 10% of the raw energy flux produced by the tide can be extracted without causing undue modification to the flow characteristics.