Fossil fuels—coal, oil, and natural gas—are carbon-based energy sources formed over millions of years from decomposed organic matter. They provide the majority of Canada's and the world's energy, powering vehicles, heating buildings, generating electricity, and providing raw materials for countless products. However, burning fossil fuels releases carbon dioxide, the primary greenhouse gas driving climate change, along with pollutants affecting air quality and health. Canada is both a major fossil fuel producer (oil sands, natural gas) and consumer. Transitioning away from fossil fuels is central to climate action, involving shifts to renewable energy, electric vehicles, building efficiency, and industrial process changes. Municipalities address fossil fuels through fleet electrification, building energy codes, transit investment, and climate action plans. The energy transition involves complex trade-offs between environmental goals, economic impacts on producing regions, and ensuring affordable energy access.