The increasing integration of intermittent renewable energy sources into microgrids presents significant challenges in reliable operation and control. This paper discusses major issues and challenges in microgrid control, reviews state-of-the-art control strategies and trends, and provides a general overview of main control principles such as droop control, model predictive control, and multi-agent systems. Microgrid control strategies are classified into three levels: primary, secondary, and tertiary. Primary and secondary levels are associated with microgrid operation, while tertiary level pertains to coordinated operation with the host grid. Each level is discussed in detail based on existing technical literature. Microgrids are clusters of loads, distributed generation (DG) units, and energy storage systems (ESSs) operated in coordination to supply electricity. They can operate in grid-connected or stand-alone modes, and handle transitions between these modes. The integration of renewable energy sources, such as wind, solar, and hydrogen, has become a priority in microgrids. Policies promoting green energy technologies, including feed-in tariffs, renewable portfolio standards, and investment tax credits, are being implemented globally. Technical challenges remain in integrating renewable DERs, including reliable and economical operation of microgrids with high penetration of intermittent generation, design of appropriate demand side management (DSM) schemes, and development of new market models for intermittent energy sources. Protection schemes must be reengineered to account for bidirectional power flows, and new voltage and frequency control techniques must be developed for power-electronics-interfaced distributed generation. The microgrid concept is an appealing alternative for overcoming challenges in integrating DER units. However, several issues remain unsolved for seamless deployment. Current efforts focus on designing special protection schemes and control systems for reliable, secure, and economical operation of microgrids in either grid-connected or stand-alone mode. This paper presents a general overview of existing technologies and remaining challenges in microgrid control. The paper is organized as follows: Section II discusses the concept of microgrids and their various evolved forms. Section III reviews the requirements of microgrid control and protection systems and discusses their most relevant challenges. Section IV presents an overview of hierarchical control systems applied to microgrids and discusses the classification of control features in different hierarchical levels. Sections V and VI review the state-of-the-art in microgrid's primary and secondary control levels, respectively. Finally, some conclusions are drawn in Section VII. Microgrids can have any arbitrary configuration, but some entities promote specific configurations. A microgrid can operate in grid-connected or stand-alone modes, and handle transitions between these modes. In grid-connected mode, the power deficit can be supplied by the main grid, while in stand-alone mode, the real and reactive power generated within the microgrid must balance the demand of local loads. Islanding, or disconnection from the host grid, can be intentional or unintentional, and proper detection is essential for safety and operation. Microgrids without a point of common coupling (PCC) are called isolatedThe increasing integration of intermittent renewable energy sources into microgrids presents significant challenges in reliable operation and control. This paper discusses major issues and challenges in microgrid control, reviews state-of-the-art control strategies and trends, and provides a general overview of main control principles such as droop control, model predictive control, and multi-agent systems. Microgrid control strategies are classified into three levels: primary, secondary, and tertiary. Primary and secondary levels are associated with microgrid operation, while tertiary level pertains to coordinated operation with the host grid. Each level is discussed in detail based on existing technical literature. Microgrids are clusters of loads, distributed generation (DG) units, and energy storage systems (ESSs) operated in coordination to supply electricity. They can operate in grid-connected or stand-alone modes, and handle transitions between these modes. The integration of renewable energy sources, such as wind, solar, and hydrogen, has become a priority in microgrids. Policies promoting green energy technologies, including feed-in tariffs, renewable portfolio standards, and investment tax credits, are being implemented globally. Technical challenges remain in integrating renewable DERs, including reliable and economical operation of microgrids with high penetration of intermittent generation, design of appropriate demand side management (DSM) schemes, and development of new market models for intermittent energy sources. Protection schemes must be reengineered to account for bidirectional power flows, and new voltage and frequency control techniques must be developed for power-electronics-interfaced distributed generation. The microgrid concept is an appealing alternative for overcoming challenges in integrating DER units. However, several issues remain unsolved for seamless deployment. Current efforts focus on designing special protection schemes and control systems for reliable, secure, and economical operation of microgrids in either grid-connected or stand-alone mode. This paper presents a general overview of existing technologies and remaining challenges in microgrid control. The paper is organized as follows: Section II discusses the concept of microgrids and their various evolved forms. Section III reviews the requirements of microgrid control and protection systems and discusses their most relevant challenges. Section IV presents an overview of hierarchical control systems applied to microgrids and discusses the classification of control features in different hierarchical levels. Sections V and VI review the state-of-the-art in microgrid's primary and secondary control levels, respectively. Finally, some conclusions are drawn in Section VII. Microgrids can have any arbitrary configuration, but some entities promote specific configurations. A microgrid can operate in grid-connected or stand-alone modes, and handle transitions between these modes. In grid-connected mode, the power deficit can be supplied by the main grid, while in stand-alone mode, the real and reactive power generated within the microgrid must balance the demand of local loads. Islanding, or disconnection from the host grid, can be intentional or unintentional, and proper detection is essential for safety and operation. Microgrids without a point of common coupling (PCC) are called isolated