[18F]-2-fluoro-2-deoxy-D-glucose positron emission tomography (FDG PET) is considered a valuable tool in the diagnosis and staging of cancer. In addition, it seems promising as a technique to monitor response to therapy. Progress is hampered, however, by the fact that various methods for the analysis of uptake of FDG in tumours have been described and that it is by no means clear whether these methods have the same sensitivity for monitoring response to treatment. As interest in monitoring response using FDG PET is growing, the danger exists that non-optimal methods will be used for evaluation. Hence an overview of the various analytical methods is given, highlighting both advantages and shortcomings of each of the methods. The ideal analytical method for response monitoring should represent an optimal trade-off between accuracy and simplicity (clinical applicability). At present, that trade-off still needs to be defined. Studies relating response, as measured with any of the available analytical methods, to outcome are urgently needed. Until then response monitoring studies should be conducted in such a way that all analytical methods can be compared with the most quantitative one, which at present is full compartmental modelling of the data.