RNA interference (RNAi) is a biological mechanism that regulates gene expression by silencing specific genes. Initially discovered in plants, RNAi is now known to occur in various organisms, including humans. The process begins when the enzyme Dicer cleaves double-stranded RNA (dsRNA) into interfering RNAs. These siRNAi encompasses various pathways: microRNAs (miRNAs) attach to the 3' UTRs of mRNA, resulting in degradation or repression; siRNAs, regardless of being endogenous or synthetic, facilitate mRNA cleavage via RISC; PIWI-interacting RNAs (piRNAs) silence transposons in germline cells; endogenous siRNAs (esiRNAs) function similarly to siRNAs; and nuclear RNAi (nRNAi) induces transcriptional silencing and heterochromatin formation. Collectively, these pathways contribute to gene regulation and genome stability, defending cells against parasitic genes, regulating gene expression during growth and development, and adapting to environmental stresses. RNAi has become an invaluable molecular biology tool for studying gene function and shows promise in developing new therapeutic approaches for various diseases.