This chapter aims to provide a comprehensive foundation for understanding lithium/sulfur (Li/S) batteries and their current research. It begins with an introduction to their fundamentals, followed by an overview of the current state of Li/S battery development and progress. Two major operation mechanisms of sulfur-based cathodes (i.e., solid-liquid-solid and full-solid mechanisms) will be discussed and compared. Key challenges hindering commercial viability of Li/S batteries will be analyzed, along with potential solutions to address these issues. By the end of this chapter, readers will have a clear understanding of the field’s landscape and the essential knowledge to embark on research in Li/S battery technology.