Gas turbine cogeneration and district energy plants can provide significant environmental improvements to the long term mix of energy production, in terms of low air pollutants and greenhouse gases. This paper is intended to discuss the fundamental need to integrate plant efficiency considerations into the environmental permitting rules to address all air issues at the same time, as described in the 1992 Canadian gas turbine emission guideline. The paper also compares the relevant air emissions, primarily nitrogen oxides (NOx) and carbon dioxide (CO2), from modern gas turbine plants, and the Canadian operating experience with cost-effective emission prevention methods.