Conventional diagnostics for cancer rely primarily on anatomical techniques. However, these techniques cannot monitor the changes at the molecular level in normal cells, which possibly signal the onset of cancer at its very earliest stages. For accurate prediction of the carcinogenesis at the molecular level, targeting ligands have been used in combination with imaging probes to monitor this biological process. Among these targeting ligands, aptamers have high binding affinity to various targets ranging from small molecules to whole organisms, and, hence, exceptional recognition ability. Many recent studies have been reported on aptamer-based molecular imaging, clearly indicating its clinical and diagnostic utility. In this review, we will discuss some key results of these studies.