This chapter focuses on PET/MRI imaging technology and its application in hepatobiliary diseases. With the shift of medical model from traditional clinical diagnosis to early warning and molecular diagnosis, molecular imaging technology has become an important means of early diagnosis and treatment of diseases. PET/MRI, as a representative of multimodal imaging technology, combines the advantages of PET’s functional imaging and MRI’s high-resolution anatomical imaging and overcomes the limitations of PET/CT in terms of soft-tissue resolution, radiation dose, and so on. Its structure includes a magnetically compatible PET detector ring, time-of-flight (TOF) technology, MRI body coil, etc. PET/MRI shows higher sensitivity and specificity in tumor diagnosis, efficacy assessment, and recurrence monitoring, and is especially valuable in the early diagnosis and individualized treatment of hepatobiliary tumors. In addition, PET/MRI has unique advantages in the diagnosis of pediatric diseases because of its lack of ionizing radiation and high soft tissue resolution.