<p>Power systems are undergoing extensive changes to pursue the goal of a carbon-free environment. In the course of these changes, the expansion of the penetration of renewable energy sources (RES) and the digitalization of demand-side electrical equipment causes a sharp increase in uncertainty and variability in power systems. As a result, it is difficult to plan and operate the system to balance supply and demand, which endangers its sustainability. To deal with these challenges in different studies, demand-side flexibility (DSF) in power systems has been suggested. Many factors, including market prices, behavioral and cultural habits of the community, etc., affect DSF. Therefore, in the literature, considering these factors, various approaches and methods have been developed to assess demand-side flexibility. In a general category, they can be divided into top-down and bottom-up approaches. In the top-down approach, the demand side is considered a large flexible resource and does not pay attention to the flexibility of the DSF resources. In the bottom-up approach, the flexibility of the DSF resources is generalized to the national or regional level. In these approaches, very big assumptions are made, which challenges the assessment results. In this article, a comprehensive overview of DSF potential and DSF assessment approaches will be done, and the requirements and methods development approaches will be proposed.</p>