Small RNAs, ranging from 20 to 30 nucleotides, play crucial regulatory roles in eukaryotic organisms by guiding processes at the DNA or RNA level. They are often derived from double-stranded or hairpin-structured single-stranded RNAs, processed by RNase III enzymes and loaded into argonaute-containing complexes. These small RNAs can cause transcriptional or post-transcriptional gene silencing through mechanisms like heterochromatin formation or mRNA degradation. They are not only derived from foreign nucleic acids but also from endogenous loci, regulating various developmental and physiological processes. This review focuses on three major classes of endogenous small RNAs in plants: microRNAs (miRNAs), trans-acting siRNAs (ta-siRNAs), and heterochromatic siRNAs (hs-siRNAs), emphasizing their roles in developmental regulation. miRNAs are processed from hairpin precursors and guide translational inhibition or mRNA cleavage. They are involved in developmental regulation, such as phase transitions and hormone biosynthesis. ta-siRNAs are generated through miRNA-mediated cleavage of non-coding transcripts and are involved in gene regulation, including auxin signaling and organ polarity. hs-siRNAs are derived from repeats and transposable elements, guiding DNA and histone methylation for transcriptional gene silencing. Argonaute proteins, which bind small RNAs, are divided into subfamilies, with AGO1 and AGO4 involved in miRNA and siRNA functions. The biogenesis of small RNAs involves RNA-dependent RNA polymerases (RdRPs) and DNA-dependent RNA polymerases (Pol II, IV, V). These enzymes produce small RNAs that are processed by Dicer-like (DCL) proteins into mature forms. Small RNAs are essential for maintaining genome integrity by silencing transposable elements and regulating gene expression. They are involved in developmental processes such as pattern formation, morphogenesis, and hormone signaling. The functions of small RNAs are regulated through feedback mechanisms and post-transcriptional modifications, ensuring their proper expression and activity in different cellular contexts.Small RNAs, ranging from 20 to 30 nucleotides, play crucial regulatory roles in eukaryotic organisms by guiding processes at the DNA or RNA level. They are often derived from double-stranded or hairpin-structured single-stranded RNAs, processed by RNase III enzymes and loaded into argonaute-containing complexes. These small RNAs can cause transcriptional or post-transcriptional gene silencing through mechanisms like heterochromatin formation or mRNA degradation. They are not only derived from foreign nucleic acids but also from endogenous loci, regulating various developmental and physiological processes. This review focuses on three major classes of endogenous small RNAs in plants: microRNAs (miRNAs), trans-acting siRNAs (ta-siRNAs), and heterochromatic siRNAs (hs-siRNAs), emphasizing their roles in developmental regulation. miRNAs are processed from hairpin precursors and guide translational inhibition or mRNA cleavage. They are involved in developmental regulation, such as phase transitions and hormone biosynthesis. ta-siRNAs are generated through miRNA-mediated cleavage of non-coding transcripts and are involved in gene regulation, including auxin signaling and organ polarity. hs-siRNAs are derived from repeats and transposable elements, guiding DNA and histone methylation for transcriptional gene silencing. Argonaute proteins, which bind small RNAs, are divided into subfamilies, with AGO1 and AGO4 involved in miRNA and siRNA functions. The biogenesis of small RNAs involves RNA-dependent RNA polymerases (RdRPs) and DNA-dependent RNA polymerases (Pol II, IV, V). These enzymes produce small RNAs that are processed by Dicer-like (DCL) proteins into mature forms. Small RNAs are essential for maintaining genome integrity by silencing transposable elements and regulating gene expression. They are involved in developmental processes such as pattern formation, morphogenesis, and hormone signaling. The functions of small RNAs are regulated through feedback mechanisms and post-transcriptional modifications, ensuring their proper expression and activity in different cellular contexts.