Noninvasive imaging in living subjects with positron emission tomography (PET) provides early detection of diseases in humans. For this application, it is necessary to prepare specific molecular imaging probes labeled with positron-emitting radioisotopes such as fluorine-18 for obtaining high-quality PET imaging. In this review, we describe recent trends in the F-18 radiolabeling method for the introduction of no-carrier-added fluorine-18, which was obtained from an $^{18}O(p,n)^{18}F$ reaction, into a specific molecular site, which in turn is intended to serve as an imaging agent for PET study. These labeling protocols are based on ionic liquid media $^{18}F$ radiofluorination in the presence of some water, enzymatic $^{18}F$ fluorination using fluorinase in water solution, non-polar protic alcohol media $^{18}F$ radiofluorination and its mechanism, and nucleophilic $^{18}F$ fluorination of an aromatic iodonium salt precursor.