Positron emission tomography (PET) is a powerful molecular imaging technology with the ability to image and monitor molecular events in vivo and in real time. With the increased application of PET radiopharmaceuticals for imaging physiological and pathological processes in vivo, there is a demand for versatile positron emitters with longer physical and biological half-lives. Traditional PET radionuclides, such as carbon-11 (11C) and fluorine-18 (18F), have relatively short half-lives (20 min and 110 min, respectively). Among the currently available positron emitters, the non-standard radiohalogen iodine-124 (124I) has the longest physical half-life at 4.2 d. This, combined with the well characterized radiochemistry of radioiodine, is contributing to the increasing utility of 124I in investigating slow and complex pharmacokinetic processes in clinical nuclear medicine and small animal PET imaging studies. This review will summarize the progress to date on the potential of 124I as a positron emitting nuclide for molecular imaging purposes, beginning with the production of 124I. Particular emphasis will be placed on the basic radiochemistry as it applies to the production of various 124I-labeled compounds, from small molecules, to biomolecules such as peptides and proteins, and finally to macromolecules like nanoparticles. The review will conclude by highlighting promising future directions in using 124I as a positron emitter in PET radiochemistry and molecular imaging.