This thesis updates estimates of agricultural water use in Ontario by applying agricultural water use coefficients to Statistics Canada 2001, 2006 and 2011 Agricultural Census Data and evaluates prior methods that have been developed to compile water estimates. This thesis also evaluates the use of crop water budgets as measures of water use by agriculture in Ontario. Furthermore this thesis examines the roles and responsibilities of the Government of Ontario in encouraging improved efficiency in water use in Ontario agriculture by examining changing perceptions of water use in Canada and Ontario, developing consistent definitions of agriculture water use, water conservation and water use efficiency, and using the existing categories of market and non-market failures as diagnostic keys to identifying problems in agricultural water use in the Ontario context.