
Industry Structure	# of Firms	Product Diff.	Entry/Exit
Perfect Competition	Many	None	Free
Monopoly	One	None	Highly Restricted
Monopolistic Competition	Many	Significant	Free
Oligopoly	Few	Some	Restricted

It is the presence of "few" firms which makes Oligopoly a fascinating industrial structure. In the other three industrial structures we studied, the behavior of individual firms could be safely ignored, either because there were no others as in Monopoly, or because there were too many for any one to matter as in Perfect and Monopolistic competition.