The existence of extensive networks for electric power transmission from distant places to consumption centers is one of the characteristics of electric grids in the world. This arrangement of the grid causes many problems, such as the loss of electrical energy in long routes, as well as threatening the stability of the grid. On the other hand, the existence of distribution grids that are fed only through these transmission grids always exposes these grids to blackouts and instability. One of the upcoming solutions to overcome these problems is to use distributed and separate grids. In fact, during disruption in the grid, the microgrid and nanogrid are separated from the upstream grid, and the island resulting from the disturbance in the power grid is isolated. This ability to create separate islands that increases reliability compared to conventional grids, and on the other hand, provides better services, improves power adequacy and high security to subscribers in smart cities. Hence, this chapter provides a comprehensive review on microgrid and nanogrid definitions. Components of these grids as well as characteristics of each components are presented. Then, applications of microgrids and nanogrids in smart cities are classified and introduced. Microgrid and nanogrid design and related issues are also discussed. International standards and codes associated with microgrid and nanogrid design and operation are introduced focusing on aims, scopes, and key technical and environmental restrictions. Finally, recent projects developed in developed countries are introduced.