MiRNAs are ~20 nt small RNAs that regulate networks of proteins using a seed region of nucleotides 2-8 to complement the 3' UTR of target mRNAs. The biogenesis and function of miRNAs as translational repressors is facilitated by protein counterparts that process primary and precursor miRNAs to maturity (Drosha/DCGR8 and Dicer/TRBP respectively) and incorporate miRNAs into the protein complex RISC to recognize and repress target mRNAs (RISC proteins: Ago/TRBP1/TRBP2/DICER). Similarly, siRNAs through comparable mechanisms are loaded into the protein complex RITS to heterochromatin formation of DNA and suppress transcription of particular genes. MiRNAs are also regulated themselves through many different pathways including transcriptional regulation, post-transcriptional RNA editing, and RNA tailing. Dysregulation of miRNAs and the protein participants that mature them are implicated in the development of a number of diseases, tumorigenesis, and arrested development of embryonic cells. In this chapter, we will explore the biosynthesis, function, and regulation of miRNAs.