MicroRNAs (miRNAs) are small non-coding RNAs that play crucial roles in gene expression regulation. Dysregulation of miRNA expression is a common feature in human cancer, influenced by various mechanisms such as amplification or deletion of miRNA genes, abnormal transcriptional control, epigenetic changes, and defects in miRNA biogenesis. MiRNAs can act as both oncogenes and tumor suppressors, affecting key hallmarks of cancer, including proliferation, evasion of growth suppressors, resistance to cell death, invasion, and angiogenesis. They have emerged as potential biomarkers for cancer diagnosis, prognosis, and therapeutic targets. This review focuses on the mechanisms by which miRNAs regulate tumor development and their potential as biomarkers and therapeutic tools.MicroRNAs (miRNAs) are small non-coding RNAs that play crucial roles in gene expression regulation. Dysregulation of miRNA expression is a common feature in human cancer, influenced by various mechanisms such as amplification or deletion of miRNA genes, abnormal transcriptional control, epigenetic changes, and defects in miRNA biogenesis. MiRNAs can act as both oncogenes and tumor suppressors, affecting key hallmarks of cancer, including proliferation, evasion of growth suppressors, resistance to cell death, invasion, and angiogenesis. They have emerged as potential biomarkers for cancer diagnosis, prognosis, and therapeutic targets. This review focuses on the mechanisms by which miRNAs regulate tumor development and their potential as biomarkers and therapeutic tools.