MicroRNAs (miRNAs or miRs) are short non-coding RNAs that down-regulate gene expression by silencing specific target mRNAs through translational inhibition and/or degradation. Interestingly, a large fraction of miRNAs maps within introns of their “host” genes, generating potentially interesting regulatory circuits, the full significance of which has barely been appreciated. In this review, we offer a comprehensive yet short treatise on the nature, location, evolution, regulation and potential mechanisms of action of the different types of intronically situated miRNAs. It is hoped that the outline presented here will provoke novel thoughts and research in this new and exciting miRNA world.