This report describes the development of Canadian water quality guidelines for the nitrate ion for the protection of aquatic life. It contains a review of technical background information on the chemical & physical properties of nitrate & nitrate salts, the nitrogen cycle, natural & anthropogenic sources, fate & behaviour of nitrate in the environment, nitrate environmental concentrations, and effects of nitrates on aquatic organisms (including fish, plants, invertebrates, and semi-aquatic mammals & birds). The final section sets out nitrate concentration guidelines to protect fresh & marine water quality and aquatic life, and provides guidance on the application of the guidelines. Appendices include tables that summarize freshwater & marine toxicity studies.