Strain glass is a frozen disordered ferroelastic state with short-range strain order only. It is a conjugate state to the long-range ordered ferroelastic state or martensite. In this chapter, recent progress in strain glass and strain glass transition is reviewed. It is shown that a strain glass bears all the features of a glass, being parallel to other types of glasses such as relaxor ferroelectrics and cluster-spin glasses. Novel properties of strain glass are demonstrated. The origin of strain glass is discussed in terms of its relation to point defects. Finally, it is shown that the insight gained from strain glass may be able to solve a number of long-standing puzzles in ferroelastic community.