The ability to observe the dynamic of RNA in single living cells offers many exciting opportunities in biology and medicine. In the last few years, molecular beacons (MBs) have shown great potential in monitoring RNA synthesis, transportation, and localization with good sensitivity and selectivity. A hairpin structure probe, MB is a dual-labeled single stranded oligonucleotide that only fluoresces in the presence of target sequences. In this paper, the basic principle and design of MB will be described. The application of MB for RNA imaging in living cells will be reviewed. The limitations of MB for in vivo application will be identified. In the last section of the article, the efforts on designing better MBs for highly sensitive and selective RNA imaging will be discussed.