Adenomyosis is a common nonneoplastic uterine disease pathologically composed of ectopic endometrial tissues and a myometrial inflammatory change. Transvaginal ultrasonography is used as a primary imaging modality for screening adenomyosis. MRI is considered a confirmative study for diagnosing adenomyosis. Frequently, adenomyosis diffusely involves myometrium, resulting in muscular hypertrophy. Less frequently, the lesion can manifest a localized form that resembles various uterine lesions including myoma, endometrial cancer, and myometrial contraction. Since adenomyosis can show various imaging features, it may cause a diagnostic dilemma. This chapter will describe imaging features of adenomyosis, unusual types or growth patterns of adenomyosis, and differential diagnoses.