Central Ontario: This region is north of the "905" belt, running from Grand Bend on Lake Huron in the west, encompassing most of Lake Simcoe in the east, and extending north along Algonquin Park to French River and Lake Nipissing. Its larger centres include Barrie, Owen Sound, Newmarket and Hunstville.

Eastern Ontario: This region runs along Lake Ontario, from Port Hope in the west, along the St. Lawrence to Quebec, and up the Ottawa River to north of Algonquin Park. Its major centres are Ottawa, Kingston, Peterborough, Pembroke, Cornwall and Belleville. It also includes the Haliburton area.

Ottawa: This grouping includes all of urban Ottawa, and extends from Cumberland in the east to Herzberg Road in the west. The southern riding of Nepean-Carleton includes Osgoode, Rideau and Goulburn. Some of the City of Ottawa lies outside this grouping, most notably the Kanata area, which is in Lanark-Carleton.

Hamilton Niagara: This riding extends along the Lake Ontario shoreline west of the Greater Toronto Area and down the Niagara Peninsula. Major urban centres include Niagara Falls, Kitchener-Waterloo, Guelph, Cambridge and Hamilton.

The "905" Belt: This region, nicknamed "905" for its telephone area code, consists of ridings that ring the city of Toronto. It extends to the communities of Clarington to the east, King City and Brampton in the north and Mississauga in the west.

Toronto: This region consists of all the ridings within the Toronto city limits. It extends from the former city of Scarborough in the east to the former city of Etobicoke in the west, north to the former city of North York and south to Lake Ontario.

Northern Ontario: This huge region encompasses everything north of French River and Lake Nipissing. It also dips below Highway 17 between Quebec and Lake Nipissing, taking in Nipissing and much of Algonquin Park. Its major centres include Sudbury, Thunder Bay and North Bay.

Southwestern Ontario: This region consists of ridings in the southwestern corner of Ontario. Its major urban centres include Windsor, London and Sarnia.