This review provides an overview of microRNA (miRNA) biogenesis, mechanisms of action, and their circulation. MiRNAs are non-coding RNAs that play crucial roles in gene regulation by interacting with target mRNAs, leading to mRNA degradation and translational repression. The biogenesis of miRNAs involves two main pathways: the canonical pathway, where pri-miRNAs are processed into pre-miRNAs by Drosha and Dicer, and the non-canonical pathways, which involve alternative processing routes. MiRNAs can also activate gene expression under certain conditions. The dynamics of miRNA action are influenced by factors such as subcellular localization, abundance, and affinity of miRNA-mRNA interactions. MiRNAs can be secreted into extracellular fluids and transported to target cells via vesicles or protein binding, where they function as chemical messengers in cell-cell communication. The review also discusses the role of miRNAs in transcriptional and post-transcriptional gene regulation within the nucleus, the dynamics of miRNA actions, and the secretion and uptake of extracellular miRNAs. Finally, it highlights the potential of extracellular miRNAs as biomarkers and signaling molecules in various biological and pathological processes.This review provides an overview of microRNA (miRNA) biogenesis, mechanisms of action, and their circulation. MiRNAs are non-coding RNAs that play crucial roles in gene regulation by interacting with target mRNAs, leading to mRNA degradation and translational repression. The biogenesis of miRNAs involves two main pathways: the canonical pathway, where pri-miRNAs are processed into pre-miRNAs by Drosha and Dicer, and the non-canonical pathways, which involve alternative processing routes. MiRNAs can also activate gene expression under certain conditions. The dynamics of miRNA action are influenced by factors such as subcellular localization, abundance, and affinity of miRNA-mRNA interactions. MiRNAs can be secreted into extracellular fluids and transported to target cells via vesicles or protein binding, where they function as chemical messengers in cell-cell communication. The review also discusses the role of miRNAs in transcriptional and post-transcriptional gene regulation within the nucleus, the dynamics of miRNA actions, and the secretion and uptake of extracellular miRNAs. Finally, it highlights the potential of extracellular miRNAs as biomarkers and signaling molecules in various biological and pathological processes.