PET using FDG has been proposed as a functional whole body imaging modality that images various types of malignancies with relatively high sensitivity and specificity in a reasonably short time. It depicts a lesion based on abnormal glucose metabolism whereas CT as a high-resolution anatomical imaging detects malignant process mostly based on altered anatomy. PET/CT combines the advantages of PET and CT, and has a great value in early detection of disease, accurate staging or restaging, early assessment of treatment response, decision on therapeutic plans, and rapid localization of recurrence. Exact anatomical localization of a lesion with increased FDG uptake by CT is considered to be the most important factor that improves the diagnostic accuracy of PET/CT. So far, limited studies have been reported using PET/CT in comparison with PET only and mainly proved additional value of PET/CT in malignant tumors in which conventional PET already had advantages over anatomical imaging. PET/CT appears to have a promising role in the field of radiotherapy planning. Another potential of PET/CT would be in the evaluation of tumors with low FDG uptake by way of CT or new PET tracers. PET/CT is in the stage of its early infancy and further studies remain to be performed to establish applications of PET/CT in clinical oncology. In this review, we will discuss the principle of PET, the background of the emergency of PET/CT, advantages, pitfalls, and debates of PET/CT along with clinical applications and future perspectives of thereof.