The environmental assessment of comprehensive plans is often referred to as 'strategic environmental assessment' (SEA). SEA facilitates the integration of environmental and sustainability considerations and comprehensive planning. The purpose of this thesis is to evaluate the effectiveness of SEA of comprehensive plans in three case studies located in Scotland, California and New Zealand. To compare effectiveness, a framework is developed against which the cases are evaluated. This study demonstrates that SEA application is only adequately effective. It was found that current SEA practice does not accurately reflect SEA theory, and furthermore, that there are a number of shortcomings identified in current practice that need to be improved. The knowledge for conducting SEA successfully is there and waiting to be applied. SEA can be seen as a catalyst toward more integrated planning for sustainable development. SEA is still evolving and current practice provides a stepping stone for something more substantial.