An eco-city is a city designed and built with a thorough consideration of environmental impacts. In eco-cities, inhabitants are dedicated to minimizing required inputs of energy, water, and food and to reducing waste outputs of heat and pollution. This paper examines how eco-city theory is applied in urban planning and design. The eco-city concept was introduced, international case studies on the eco-cities in developed countries were explored using case study methods to determine the application of eco-city concepts and theories, and a set of recommendations for converting Cairo, Egypt into an eco-city were formulated. Results showed that the eco-city concept can be effectively applied in combination with the characteristics specific to a locality. The study concludes that eco-city theory can be effectively applied in urban planning and design.