MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression either by mediating translational repression or reducing the stability of a target mRNA. Deregulated expression of miRNAs is a common feature of human cancers and a growing body of evidence demonstrates the role of miRNAs as either oncogenes or tumor suppressors. Many miRNAs have been reported to be associated with the pathogenesis of primary prostate cancer (PCa) and the development of castration resistant prostate cancer (CRPC). PCa is the most common cancer and second leading cause of cancer death in American men. Although patients with primary PCa can be treated with chemotherapy and hormone therapy, many of them will develop resistance to conventional therapies and progress to a more sever condition called CRPC, which remains one of the most difficult cancers to treat. Since emerging evidence suggests miRNAs’ significant roles in the tumorigenesis of primary PCa and CRPC, the potential of using miRNAs as drug targets and biomarkers for primary PCa and CRPC has been gaining more attention. The aim of this review is to summarize recent studies on the involvements and mechanisms of the actions of several miRNAs in the development and progression of primary PCa and CRPC. Additionally, the potentail applications of using miRNAs as biomarkers and drug targets are briefly discussed.