Hybrid energy systems have been identified as a viable mechanism to address the limitations of single-source renewable energy production. In view of this, several research works have been carried out to determine the optimal mix of different renewable energy resources. This paper presents a comprehensive review of the optimisation approaches proposed and adopted by various authors in the literature for optimal sizing of hybrid energy system. It is observed that the objective functions considered by a large percentage of researchers to opmise the sizing of hybrid renewable energy systems are cost minimization, system reliability enhancement and environmental pollution reduction. Basically, the existing works are broadly classified into technical, economic and environmental factors. The parameters considered, among others in the review, include cost of energy, system reliability enhancement and emissions reduction. Other factors covered in the literature are equally discussed in this article. Similarly, simulation and optimisation software used for the same purpose are equally covered in the paper. In essence, the main aim of this paper is to provide a scope into the works that have been carried out in the field of renewable energy with the view to informing researchers and members of the public alike, on trends in methods applied in optimal sizing of hybrid renewable energy systems. The information provided in this work is very crucial in advancing research in the field.