<p>MicroRNAs (miRNAs) are ~ 22-nucleotide-long noncoding RNAs that regulate gene expression at the post-transcriptional level through mRNA cleavage or translational repression. To date, approximately 2,700 mature human miRNAs have been identified, although biological functions have been assigned to fewer than 600. Among these, miR-155 has emerged as a regulatory miRNA with important roles in metabolism, immune homeostasis, and disease pathogenesis. Dysregulated miR-155 expression has been implicated in metabolic disorders, including diabetes and preeclampsia, as well as autoimmune diseases such as rheumatoid arthritis, multiple sclerosis, and systemic lupus erythematosus. Mechanistic and preclinical evidence indicates that aberrant miRNA-155 expression contributes to tumorigenesis by modulating tumor suppressor genes in hematologic malignancies and solid tumors. This review summarizes the current updates on the physiological and pathological roles of miRNA-155.</p>