While many companies may choose to participate in the IESO-administered markets, any company with equipment directly connected to the IESO-controlled grid to convey electricity into, through, or out of the grid must become a market participant.

Transmitters own the equipment that makes up the IESO-controlled grid. Transmitters do not buy or sell energy; rather, they add value by creating and maintaining the grid that connects generators and wholesale loads throughout the province.

Transmitters will be compensated by an approved uplift (overhead) charge applied to all electricity purchased through the IESO-administered market. Hydro One is an example of a transmitter.

Distributors are the local distribution companies (LDC) which take electricity from the IESO-controlled grid and distribute it to retail consumers. Distributors deliver electricity directly to retail consumers at the appropriate voltage for their needs and are compensated by payments made to them by their own customers.

The Ontario Energy Board is responsible for issuing licences and regulations for distributors.

Generators produce electricity which is then transmitted over the grid, stepped down in voltage at transformer stations, and then distributed to consumers. There are two types of generators -- dispatchable and non-dispatchable. Most of the energy supply in Ontario is provided by dispatchable generators.

Loads are the consumers of energy in Ontario. Non-dispatchable loads draw electricity from the grid as needed, while dispatchable loads can adjust their consumption in response to instructions from the IESO.

Many companies participate in the IESO-administered markets without having physical facilities that produce or consume electricity. These companies may participate in both the real-time markets and the financial markets.

Wholesalers buy energy on the wholesale market, and sell energy and services to other market participants. Retailers sell energy and services to consumers at the retail level (that is, they may sell to non-market participants). Both wholesalers and retailers are re-selling electricity rather than producing electricity themselves. They may also act as importers or exporters. Importers bring energy products into Ontario from one of the five neighbouring jurisdictions: Quebec, Manitoba, Michigan, Minnesota and New York. Exporters export electricity from Ontario into neighbouring jurisdictions.