Sarcoidosis is a complex and intriguing disease that has puzzled medical researchers for decades. This chapter provides a comprehensive definition of sarcoidosis, outlining its fundamental characteristics and the mechanisms underlying its development. At its core, sarcoidosis is an inflammatory condition marked by the presence of granulomas—tiny clusters of immune cells that can form in various organs and tissues throughout the body. Most commonly, these granulomas are found in the lungs and lymph nodes, but they can also affect the skin, eyes, heart, and other organs. Despite extensive research, the precise cause of sarcoidosis remains elusive, though it is widely believed to be the result of an abnormal immune response to an unknown trigger.