MicroRNAs (miRNAs) have emerged as a novel class of endogenous, small, noncoding RNAs that negatively regulate gene expression via degradation or translational inhibition of their target mRNAs. Over 700 miRNAs have been identified and sequenced in humans, and the number of miRNA genes is estimated at more than 1000. Individual miRNA is functionally important as a transcription factor because it has the ability to regulate the expression of multiple genes through binding to its target with imperfect or perfect complement. In the heart, miRNAs have been involved in several clinical scenarios, such as ischemia/reperfusion (I/R) injury and heart failure suggesting that regulation of their function could be used as a novel cardioprotective strategy. In particular, miRNA-1, miRNA-21, miRNA-24, miRNA-29, miRNA-92a, miRNA-126, miRNA-133, miRNA-320, miRNA-199a, miRNA-208, and miRNA-195 have been shown to be regulated after I/R injury. Because tissue miRNAs can be released into circulating blood, they also offer exciting new opportunities for developing sensitive biomarkers, including miRNA-1, miRNA-126, miR-208, and miRNA-499, for acute myocardial infarction and other cardiac diseases.