## 1 Introduction  
The energy demand in the last few decades has increased drastically due to increased population, high living standards, and infrastructural development. As a result, high penetration of traditional generating units and better efficiency of a power system are required. However, the depleting fossil fuels, high consumption rate, environmental implications of traditional energy sources, inefficiency, and aging of power systems have created an unsustainable system that needs to be modified. In this context, the popularity of renewable energy sources has increased over the years [1]. Renewable energy sources include solar, wind, tidal energy, biofuel, fuel cells, and small hydro plants free from CO\({}_{2}\) emissions and available abundantly. They are also called distributed energy resources, as they are usually connected at the distribution level. The integration of distributed energy resources with a utility grid is an attractive solution, but reliable cost-effective integration still remains a challenge. The smart grid is the most promising solution as a next-generation power system encompassing clusters of interconnected micro-grids [2]. A microgrid is a combination of a low voltage distribution system with distributed generation, energy storage systems, and loads. It is the most excellent solution to achieve the decentralization of a power system. The inherent flexibility of a microgrid to be connected/disconnected from the main grid gives an improved system with reliability, less investment cost, the green effect due to renewable sources, improved power quality of a system, and reduces the losses of distribution network. A microgrid can be divided into three major groups based on topology, namely, AC, DC, and hybrid. AC microgrid is the most used configuration which incorporates existing grid infrastructure, protection, and technologies. However, the need for synchronization of distributed generation (DG) units and losses resulting from reactive power circulation are its drawbacks. Moreover, the presence of DC sources presents the problem of conversion into AC using converters which decreases the overall efficiency. The increased penetration of DC-based distributed energy resources such as PV and fuel cells, energy storage systems, and loads has paved the way for DC microgrids. However, it requires a considerable modification of the available power network. Both individual AC and DC microgrids suffer from inefficiency due to multiple DC/DC, AC/DC, and DC/AC conversions. A hybrid microgrid is an excellent solution for the realization of smart grids in conventional distribution networks. It combines the advantages of both AC and DC microgrids allowing direct integration of AC and DC-based distributed energy resources, energy storage systems and loads, no synchronization, simple transformation, and economic feasibility [3; 4; 5; 6]. Among the various meanings of hybrid microgrid, we mainly intend \"hybrid microgrid\" as \"an integrated AC/DC low voltage microgrid system\" or \"a hybrid low-voltage microgrid, unless otherwise specified.\"  
The hybrid microgrid comprises vast areas for research. For any further research, a detailed literature review regarding the latest developments in the field of a hybrid microgrid is required. The review papers make this task easier by summarizing the studies and enabling the reader to identify the existing research gaps. This paper collectively provides a review of research regarding all the major areas of hybrid microgrids, starting from the topology and leading to optimization techniques and control methodologies employed in it. The uniqueness of this review paper is that it provides a comprehensive comparison of this review paper with other reviews of the hybrid microgrid written regarding all the major sections, e.g., challenges, optimization, power generation, and sharing control. This makes it the ultimate paper that covers the essential information for the researchers to traverse the research horizons further.