MicroRNAs (miRNAs) are a class of small non-coding RNAs that regulate gene expression at the posttranscriptional level by either degrading or blocking translation of messenger RNA targets. Recent expression profiling studies have revealed that miRNAs play important regulatory roles in a variety of cellular functions as well as in every cancer type studied so far. Lately, the discovery of miRNAs in the serum of cancer patients opened up the exciting prospective of using miRNAs as powerful and non-invasive cancer biomarkers. In this article, we review the current literature on serum miRNAs in different cancer types and the approaches used to detect and quantify these molecules. We then discuss the potential of miRNA biomarkers to improve disease diagnosis by distinguishing healthy from malignant tissues, identifying the tissue of origin in poorly differentiated tumors or tumors of unknown origin and distinguishing between different subtypes of the same tumor. We will also compare the advantages and disadvantages of potential serum miRNA biomarker molecules for cancer classification, estimation of prognosis and prediction of therapeutic efficacy. Finally, we will establish a set of criteria that these new molecules and clinical studies that use them must fulfill before they can be used as reliable tools in diagnostic and prognostic settings.