Non-invasive molecular imaging of cancer by means of the hybrid imaging modalities PET/CT and PET/MRI represents a powerful diagnostic tool in modern nuclear medicine. Radiotracers labeled with the prominent positron-emitter fluorine-18 are routinely used to target and visualize distinct biological structures dysregulated in the progression of cancer. Such tracers are therefore capable of detecting oncological pathologies in vivo at the cellular and subcellular levels in a timely manner and are thereby used for early detection of cancer as well as monitoring for treatment response in a personalized manner. This chapter describes a variety of important 18F-labeled radiopharmaceuticals that are frequently used in oncological PET imaging. Small-molecule and low-molecular-weight radiotracers for the detection of glucose utilization, amino acid transport, membrane lipid synthesis, cell proliferation, cell death, hypoxia, estrogen receptor status, DNA repair, tumor stroma, and bone mineralization of tumors are introduced. Radiotracers targeting the prostate-specific membrane antigen (PSMA) and the somatostatin receptor subtype 2 (SSTR2) expression are introduced as diagnostic companions in the important field of oncological radionuclide theranostics. The structural properties, common radiochemical synthesis approaches, as well as in vivo metabolism and accumulation mechanisms of the clinically most important 18F-labeled radiotracers are described.