Epigenetics is the study of meiotically and mitotically heritable changes in gene expression that are not coded for in the underlying DNA sequence. The term epigenetics is derived from epi- (meaning upon) and genetics. Epigenetic regulation of mammalian gene expression has profound effects in controlling cell growth, differentiation and cancer development. The important epigenetic mechanisms include DNA cytosine methylation, histone modifications and the more recently discovered non-coding RNAs. Non-coding RNAs are functional RNA molecules that are not translated into proteins. The main classes of regulatory non-coding RNAs include siRNA, miRNA, piRNA and long non-coding RNAs. There is growing evidence that regulatory non-coding RNAs play essential roles in the regulation of gene expression and are impor-tant in mammalian development and disease processes. Here, we review current research efforts aimed at understanding non-coding RNAs and their mechanisms of function in mammalian cells.