microRNAs (miRNAs) are a class of highly conserved small non-coding RNAs that negatively regulate gene expression post-transcriptionally. The emerging field of miRNA biology has begun to unravel roles for these regulatory molecules in a range of biological functions, including cardiac and skeletal muscle development, as well as in muscle-related disease processes. In this paper, we review the role of miRNAs in muscle biology. Recent genetic studies have demonstrated that miRNAs are required for both proper muscle development and function, with crucial roles for miRNAs being identified in regulating muscle cell proliferation and differentiation. Furthermore, dysregulated expression of miRNAs has been correlated to certain muscle-related diseases, including cardiac hypertrophy, cardiac arrhythmias, and muscular dystrophy.