MicroRNAs (miRNAs) are evolutionarily conserved, endogenous, noncoding small RNAs that act as post-transcriptional gene regulators. Experimental evidence has shown that miRNAs can play roles as oncogenes or tumor suppressor genes, suggesting their contribution to cancer development and progression. Expression profiles of human miRNAs demonstrated that many miRNAs are deregulated in cancers and are differentially expressed in normal tissues and cancers. Therefore, miRNA profiling is used to create signatures for a variety of cancers, indicating that the profile will help further establish molecular diagnosis, prognosis and therapy using miRNAs. This paper introduces the aberrant expression of miRNAs in human cancer, and discusses the potential of these miRNAs as biomarkers and targets/molecules for molecular therapy.