The Heat Warning and Information System (HWIS) is a coordinated provincial system that provides a consistent approach for processing and issuing heat warnings in Ontario, in order to better protect residents, vulnerable community members and visitors during the summer season. It was developed jointly by Environment and Climate Change Canada, Health Canada, the Ontario Ministry of Health and Long-Term Care, and Public Health Ontario, in consultation with provincial health units. The HWIS provides criteria for issuing heat warnings. These criteria were selected after an extensive review of epidemiological evidence about the links between temperature, humidity, and health outcomes including mortality and illness.