The chapter introduces the fundamentals of solar pond technology as a thermal storage process. It includes a brief description of solar pond technology in the context of energy storage, history, fundamentals; a classification of the different ponds used for capture and storage of solar energy; the fundamentals of the salinity gradient solar pond, describing the research efforts over recent decades; the design, construction, and the establishment of the salinity gradient, operation, and control; and finally the heat extraction systems. A discussion of investment, construction, and operational cost is also presented together with a review of the different applications in which solar ponds have the potential to supply low-grade thermal energy