microRNAs (miRNAs) are small non-coding RNAs that control gene expression posttranscriptionally and are part of the giant non codifying genoma. Cumulating data suggest that miRNAs are promising potential biomarkers for many diseases, including cancer. Prostate cancer (PCa) detection is currently based in the serum prostate-specific antigen biomarker and digital rectal examination. However, these methods are limited by a low predictive value and the adverse consequences associated with overdiagnosis and overtreatment. New biomarkers that could be used for PCa detection and prognosis are still needed. Recent studies have demonstrated that aberrant expressions of microRNAs are associated with the underlying mechanisms of PCa. This review attempts to extensively summarize the current knowledge of miRNA expression patterns, as well as their targets and involvement in PCa pathogenesis. We focused our review in the value of circulating and urine miRNAs as biomarkers in PCa patients, highlighting the existing discrepancies between different studies, probably associated with the important methodological issues related to their quantitation and normalization. The majority of studies have been performed in serum or plasma, but urine obtained after prostate massage appears as a new way to explore the usefulness of miRNAs. Large screening studies to select a miRNA profile have been completed, but bioinformatics tools appear as a new approach to select miRNAs that are relevant in PCa development. Promising preliminary results were published concerning miR-141, miR-375 and miR-21, but larger and prospective studies using standardized methodology are necessary to define the value of miRNAs in the detection and prognosis of PCa.