The Provincial Highway Network consists of all the roads in Ontario maintained by the Ministry of Transportation of Ontario (MTO), including those designated as part of the King's Highway, secondary highways, and tertiary roads. Components of the system—comprising 16,900 kilometres (10,500 mi) of roads and 2,880 bridges —range in scale from Highway 401, the busiest highway in North America, to unpaved forestry and mining access roads. The longest highway is nearly 2,000 kilometres (1,200 mi) long, while the shortest is less than a kilometre. Some roads are unsigned highways, lacking signage to indicate their maintenance by the MTO; these may be remnants of highways that are still under provincial control whose designations were decommissioned, roadway segments left over from realignment projects, or proposed highway corridors.

The Ministry of Transportation (MTO) is the provincial ministry of the Government of Ontario that is responsible for transport infrastructure and related law in Ontario, Canada. The ministry traces its roots back over a century to the 1890s, when the province began training Provincial Road Building Instructors. In 1916, the Department of Public Highways of Ontario (DPHO) was formed and tasked with establishing a network of provincial highways. The first was designated in 1918, and by the summer of 1925, sixteen highways were numbered. In the mid-1920s, a new Department of Northern Development (DND) was created to manage infrastructure improvements in northern Ontario; it merged with the Department of Highways of Ontario (DHO) on April 1, 1937. In 1971, the Department of Highways took on responsibility for Communications and in 1972 was reorganized as the Ministry of Transportation and Communications (MTC), which then became the Ministry of Transportation in 1987.