Is equating market share to market power a sound economic principle?

There is a long history of mapping market structure into market power in economic analysis. This paper addresses the validity of this principle for both homogenous and differentiated products industries. While mapping market share dominance into market power may be acceptable for homogenous goods as a rule of thumb, it is by no means a robust result. In the case of differentiated products industries, there is no theoretical foundation for such a mapping. This paper highlights the need to move towards a structural approach to assessing market power in industries.