MicroRNAs (miRNAs) and short interfering RNAs (siRNAs) regulate eukaryotic mRNA translation and degradation. These small RNAs, derived from endogenous or exogenous sources, influence mRNA stability and translation by interacting with the RNA-induced silencing complex (RISC). miRNAs and siRNAs can target mRNAs for degradation or translation repression, often through mechanisms involving endonuclease cleavage, mRNA decapping, or translational inhibition. The RISC complex, containing Argonaute proteins, plays a central role in these processes. miRNAs can direct endonucleolytic cleavage of mRNAs, leading to their degradation by cellular machinery. Additionally, miRNAs can target mRNAs for slicer-independent decay by promoting their accumulation in P-bodies, where they are subject to decapping and degradation. miRNAs also repress translation by inhibiting initiation factors or sequestering mRNAs in P-bodies, reducing their availability for ribosome binding. The interaction between miRNAs and their targets is highly specific, with base-pairing and sequence conservation playing key roles. The localization of miRNAs, RISC, and target mRNAs in P-bodies suggests that miRNAs may enhance mRNA decay by facilitating their access to decapping enzymes. The mechanisms by which miRNAs regulate mRNA metabolism are complex and involve interactions with various RNA-binding proteins and cellular pathways. Understanding these mechanisms is crucial for elucidating the broader role of miRNAs in gene regulation and cellular function.MicroRNAs (miRNAs) and short interfering RNAs (siRNAs) regulate eukaryotic mRNA translation and degradation. These small RNAs, derived from endogenous or exogenous sources, influence mRNA stability and translation by interacting with the RNA-induced silencing complex (RISC). miRNAs and siRNAs can target mRNAs for degradation or translation repression, often through mechanisms involving endonuclease cleavage, mRNA decapping, or translational inhibition. The RISC complex, containing Argonaute proteins, plays a central role in these processes. miRNAs can direct endonucleolytic cleavage of mRNAs, leading to their degradation by cellular machinery. Additionally, miRNAs can target mRNAs for slicer-independent decay by promoting their accumulation in P-bodies, where they are subject to decapping and degradation. miRNAs also repress translation by inhibiting initiation factors or sequestering mRNAs in P-bodies, reducing their availability for ribosome binding. The interaction between miRNAs and their targets is highly specific, with base-pairing and sequence conservation playing key roles. The localization of miRNAs, RISC, and target mRNAs in P-bodies suggests that miRNAs may enhance mRNA decay by facilitating their access to decapping enzymes. The mechanisms by which miRNAs regulate mRNA metabolism are complex and involve interactions with various RNA-binding proteins and cellular pathways. Understanding these mechanisms is crucial for elucidating the broader role of miRNAs in gene regulation and cellular function.