An important component of modern landfills is the liner system for the prevention of leachate contamination of surrounding ground. Among landfill liner systems, geosynthetic clay liner (GCL) has gained widespread popularity across the world because of its lower hydraulic conductivity as well as its ability to self-heal local damage, which is almost unavoidable in the field. Over the past few decades, numerous studies have been conducted to examine the performance of GCLs, particularly in regard to hydraulic conductivity, chemical compatibility, water-swelling, self-healing capacity, diffusion characteristics, gas migration, and mechanical behavior. In this paper, a brief introduction on modern GCL products is firstly given. Subsequently, the main findings of previous publications on the critical properties influencing the long-term performance of GCLs are summarized in a comprehensive manner. Finally, further research perspectives on polymer-treated GCLs are presented. This paper provides general insights that help readers gain a state-of-the-art overview of GCLs and trends for future development.