In the present review the reader will be led across the most relevant observations that prompted oncologists and haematologist to consider FDG-PET/CT as a new paradigm for FL management in clinical practice. The contribute of functional imaging for lymphoma staging, restaging, prognostication, and tumour burden definition before radio-chemo-immunotherapy will be reviewed in detail. Moreover, a special focus will be addressed to technical and practical aspects of PET scan reporting, which have been set during the last decade in order to ensure the reproducibility of the therapeutic results. Finally, the predictive role of PET/CT on long-term treatment outcome will be compared with other well-known prognosticator as minimal residual disease (MRD) detection by gene rearrangement assessment by molecular biology.