Aptamers are short single stranded nucleic acid molecules, isolated from a pool of nucleic acid molecules by the method called SELEX (Systematic Evolution of Ligands by EXponential Enrichment) that can bind to the target with high affinity and specificity. A modified SELEX method where the targets are living whole cells is referred as cell-SELEX, and is of great interest because it opens up a simple and effective method for disease diagnostics and therapeutics. The cell-SELEX based approach and recent developments of the technique for identifying aptamers as well as their application in cancer diagnostics and therapeutics are summarized here.