Osteoporosis is the most common metabolic bone disease, which refers to a state where fractures are likely to occur due to decreased bone strength. Bone strength is related to both bone quantity and bone quality, and these two factors are important in evaluating bone strength and fracture risk. Among them, the variable mainly used to represent bone quantity is bone density, which is known to determine 60–90% of bone strength, so bone density has a close relationship with bone strength, and therefore, it can be said to be the most important factor in the diagnosis of osteoporosis. In this chapter, we will learn about the concept and importance of osteoporosis, the diagnosis of osteoporosis using DXA, and the imaging findings of osteoporosis appearing in the other imaging techniques.