The article reviews the biogenesis and functions of three major classes of endogenous small RNAs in plants: microRNAs (miRNAs), trans-acting siRNAs (ta-siRNAs), and heterochromatic siRNAs. Small RNAs, which are 20–30 nucleotides long, play crucial roles in regulatory processes at the DNA or RNA level in eukaryotic organisms. They are processed from double-stranded or single-stranded RNAs with local hairpin structures by RNase III enzymes and loaded into argonaute-protein-containing effector complexes (RISCs). The article highlights the diverse mechanisms by which small RNAs guide gene silencing, including transcriptional and posttranscriptional gene silencing. It also discusses the involvement of specific proteins and polymerases in the biogenesis of these small RNAs, such as RNA polymerases II, IV, and V, and Dicer-like proteins. The developmental roles of miRNAs and ta-siRNAs are explored, including their functions in phase transitions, hormone biosynthesis and signaling, pattern formation, and morphogenesis. Additionally, the article examines the role of heterochromatic siRNAs in maintaining genome integrity and regulating gene expression. The review emphasizes the importance of small RNAs in plant development and their diverse regulatory functions.The article reviews the biogenesis and functions of three major classes of endogenous small RNAs in plants: microRNAs (miRNAs), trans-acting siRNAs (ta-siRNAs), and heterochromatic siRNAs. Small RNAs, which are 20–30 nucleotides long, play crucial roles in regulatory processes at the DNA or RNA level in eukaryotic organisms. They are processed from double-stranded or single-stranded RNAs with local hairpin structures by RNase III enzymes and loaded into argonaute-protein-containing effector complexes (RISCs). The article highlights the diverse mechanisms by which small RNAs guide gene silencing, including transcriptional and posttranscriptional gene silencing. It also discusses the involvement of specific proteins and polymerases in the biogenesis of these small RNAs, such as RNA polymerases II, IV, and V, and Dicer-like proteins. The developmental roles of miRNAs and ta-siRNAs are explored, including their functions in phase transitions, hormone biosynthesis and signaling, pattern formation, and morphogenesis. Additionally, the article examines the role of heterochromatic siRNAs in maintaining genome integrity and regulating gene expression. The review emphasizes the importance of small RNAs in plant development and their diverse regulatory functions.