This chapter describes the upcoming technology for electrical power systems that gives the appropriate solution for the integration of the distributed energy resources. In this chapter, different categories of smart grids have been classified, and the advantages, weakness, and opportunities of each one, are given in addition to determining its own operating conditions. Micro-grids are the most common kind of smart grid. It has been classified under different criteria, such as architecture with different topology (connected mode, island mode, etc.) and demand criteria (simple micro grids, multi-DG, utility) and by capacity into simple micro-grid, corporate micro-grid, and independent micro-grid, and by AC/DC type to DC micro-grids, AC micro-grids, Hybrid micro-grids. Finally, most familiar Micro-grid components have been discussed such as an energy management system along with several types of control and communication systems in addition to the economic study of a micro-grids.