As part of its program to ensure that the surface waters of Ontario are satisfactory for aquatic life and recreation, the Ontario Ministry of the Environment develops Provincial Water Quality Objectives (PWQOs) and Guidelines (PWQGs). This report details the procedures used to develop PWQOs and PWQGs. PWQOs are developed from the best available knowledge on the effects of substances on aquatic life. Data are required for acute and chronic toxicity, bioaccumulation and mutagenicity. Also considered is available information on environmental fate, physical-chemical properties, taste and odour in water and tainting of fish tissues. If adequate information is unavailable for setting a PWQO, a PWQG is developed using uncertainty factors which depend on the quantity and quality of the toxicological data.