[18F-]FDG PET(-CT) is a primarily quantitative imaging technology that is rapidly gaining ground in clinical oncology; initially for staging and diagnosis, and now increasingly as a biomarker of response to therapy. In spite of 20 years of clinical research, there is discussion about its implementation among clinicians, decision-makers and other parties about its implementation. To some extent, this relates to heterogeneity of the PET results and of trial designs, but also to differences in levels of evidence required by various parties. With PET, biological and quantitative imaging is entering the clinical domain. The current subjective perspective reviews these aspects to help clinicians understand biological and physical elements underlying [18F-]FDG PET to increase the clinical awareness of its potential and limitations.