Rapid migration from rural to urban areas leads to the emergence of urbanization and sustainability problems. Management and monitoring of resources and infrastructures are getting more important today in crowded cities. Energy consumption is increasing with the growing population and intensified in highly populated parts of cities. This increased energy consumption results in high energy demand as well as the production of more pollutants and heat in these city parts. The management of pollution and heat problems leads to an additional energy demand. Therefore, energy efficiency is becoming central challenge for urban life similar to energy efficient structuring of living organisms. Practically, all activities arranging city life require energy. For instance, job activities, transportation, security, climate, catering, entertainment, commerce etc. In this study, we review active and passive approaches, which can be utilized for improvement of energy efficiency and thus sustainability of urbanization in today's cities. We give a discussion to present a vision for future smart cities in term of improvement of energy utilization and efficiency, and relieve of environmental issues. These approaches can be helpful for future cities for relieving issues originating from population increase.