Molecular imaging using 18F-fluoro-deoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) has been successfully used in the diagnosis, initial staging, and response assessment of various malignant tumors. Advances in radiation therapy planning and delivery have ushered in the era of high-precision conformal radiotherapy allowing generation of dose distributions that conform closely to the shape of the target volume while minimizing high-dose regions in the surrounding normal tissues. Traditionally, radiation therapy planning has relied heavily on CT imaging, but recent times have witnessed tremendous enthusiasm for the use of PET/CT-guidance in radiotherapy planning. This has been largely stimulated by widespread availability and integration with treatment planning systems. However, several issues need to be addressed and challenges overcome to realize the full potential of this exciting technology. Integrating PET/CT fusion imaging into routine clinical practice can be challenging due to technical, administrative, financial, geographic, and personnel issues. Concerted efforts are urgently needed for the development of guidelines for appropriate application of this technology using standardized methodology. There is accumulating evidence that incorporating PET/CT imaging in radiotherapy planning for lung cancer, head and neck cancer and cervical cancer has a significant impact. This review highlights the promises and pitfalls of PET/CT imaging in radiotherapy treatment planning with a critical appraisal of the current best evidence for its application in the modern radiotherapy clinic, and provides a sneak preview into the future of such technology.