Noncoding RNAs (ncRNAs) have emerged as critical regulators of gene expression, genome organization, and cellular homeostasis, challenging the earlier view of noncoding regions as “junk DNA.” This chapter provides a systematic overview of the classification, biogenesis, structural features, and functional roles of ncRNAs, focusing on plants. ncRNAs are mainly categorized into small (<200 nt) and long (>200 nt) classes, including microRNAs, small interfering RNAs, PIWI-interacting RNAs, small nucleolar RNAs, long noncoding RNAs, circular RNAs, and others. Particular emphasis focuses on the roles of ncRNAs in transcriptional and posttranscriptional regulation, epigenetic modification, genome stability, nuclear architecture, development, and stress responses. Structural motifs, evolutionary conservation, and posttranscriptional modifications are highlighted as key determinants of ncRNA function. Through specific examples, this chapter underscores the regulatory versatility and evolutionary significance of ncRNAs in shaping the complexity of eukaryotic genomes.