<u>Monopoly:</u> There is a single seller in the market
<u>Oligopoly:</u> every company in this market structure is aware of the actions of the other companies (oligopolies are a small number of companies controlling the markets- there are elements of collusion in this structure because the firms work together to control prices and the market)
<u>Perfect Competition:</u> There are no barriers to entry (lots and lots of competing companies that each have a small share of the market)
<u>Collusion:</u> 3 companies secretly enter into a price agreement (this is illegal in many cases)