MicroRNAs (miRNAs) belong to the group of non-coding, small RNAs (19 to 21 nucleotide-long oligonucleotides) that are endogenously produced from their own genes. miRNAs interfere with or affect the transcription or translation of other genes, resulting in gene silencing or activation by a process known as RNAi or RNA activation, respectively. The discovery of miRNAs and related small RNAs within the last 10 years has opened up a new field in biology, called RNomics, which investigates differences in RNA populations and offers great potential in correcting the regulation of abnormally expressed and disease-causing genes, as well as in the development of novel therapeutic agents. This review focuses on the developments in miRNAs and their potential therapeutic applications.