Antisense oligonucleotides, in short antisense, are small chains of nucleic acids capable to bind to cellular ribonucleic acid (RNA) by a hybridization mechanism. In vitro, antisense are widely used as reagents to detect or block specific RNA sequences. The use of antisense as in vivo diagnostic agents is attractive because it would bring molecular imaging at the level of gene expression. However, oligonucleotides are non-canonical radiopharmaceuticals and much progress is needed to adapt them to in vivo imaging. The requirements to reach this goal include improvements in radiosynthesis, stability, targeting, and specific and non-specific binding. They will be examined in this review together with the current achievements in the applications of antisense as nuclear medicine radiopharmaceuticals.