In the past decade, energy storage systems (ESSs) as one of the structural units of the smart grids have experienced a rapid growth in both technical maturity and cost effectiveness. These devices propose diverse applications in the power systems especially in distribution networks. Despite offering numerous applications, the ESSs are new devices characterized by high investment costs. Besides technological advancement, optimal ESS planning and scheduling is one of the effective ways to reduce the costs and justifying high investment costs by taking their benefits out as much as possible. During the past few years, various studies have been conducted by the researcher to address the problem of optimal ESS planning in distribution networks. In this context, various models, methods, and considerations have been proposed to enhance the functionality of optimal planning process. The aim of this paper is to review the problem of optimal ESS planning including optimal bus location, power rating, and energy capacity determination in the distribution networks. In order to facilitate continuing and growing research in this field, a comprehensive literature survey and classification of the related studies followed by research gaps and future opportunities is provided.