MicroRNAs (miRNAs) are a class of highly conserved, small non-coding RNAs that regulate gene expression post-transcriptionally. The emerging field of miRNA biology has begun to reveal roles for these regulatory molecules in a wide range of biological processes. Dysregulated miRNA expression has been correlated to diseased hearts in human patients, whereas inhibiting the maturation of miRNAs conditionally in murine hearts has revealed that miRNAs are essential for cardiac development and function. Moreover, genetic studies have identified distinct roles for specific miRNAs during cardiogenesis, cardiac hypertrophy and electrical conduction. These previously unrecognized relationships shed new light on the regulatory mechanisms underlying heart development and pathology and suggest the potential importance of miRNAs as diagnostic markers and therapeutic targets for cardiovascular disease.