Clay is a very important material in geotechnical engineering, because it is often observed in geotechnical engineering practice. Generally, this soil type has numerous problems due to its low strength, high compressibility and high level of volumetric changes. Clay needs to be improved before it can be used in road construction, dams, slurry walls, airports and waste landfills. Improved gradation, a reduction in plasticity and swelling potential, as well as an increase in strength and workability, generally improve the stability of clay. Clay is a fine-grained soil, but not all fine-grained soils are clay. Clay minerals are very electrochemically active; thus, they affect soil microstructures. Due to these characteristics, many important soil problems related to clay have been observed in the past, the importance of which is understood. In this chapter, the properties of clay, as well as the use of clay in geotechnical engineering and geotechnical studies on clay, are examined.