Monopoly and oligopoly are the structures of a market when there is not a perfect competition. Monopoly can be defined as the market condition when a single firm or company produces products that have no alternatives, on the other hand, in oligopoly a small number of large firms produces similar or slightly different goods. In both the market conditions, there are entry barriers that prevent other new firms from competing with them. Monopoly can occur in a particular region but oligopoly occurs in larger geographical areas. There are antitrust laws that prevent firms from the restraint of trade and transacting mergers which causes a decrease in competition.