MicroRNAs (miRNAs) are small noncoding RNAs that act as post-transcriptional repressors of gene expression in organisms ranging from plants to humans. A widespread role for miRNAs in diverse molecular processes driving the initiation and progression of various tumor types has recently been described. Here, we discuss the etiology of the aberrant expression of miRNAs in human cancers and their role in tumor metastasis, which might define miRNAs as oncogenes or tumor suppressors. Moreover, we highlight the genomic/epigenetic alterations and transcriptional/post-transcriptional mechanisms associated with the misexpression of miRNAs in cancer. A better understanding of miRNA biology might ultimately yield further insight into the molecular mechanisms of tumorigenesis and new therapeutic strategies against cancer.