A large family of small 18-25 nucleotide long non coding RNA molecules now known as microRNA (miRNA) was described two decades ago, and has been recently es- tablished as post-transcriptional gene regulators. miRNAs were shown to be involved in the regulation of diverse phys- iological and developmental processes. Moreover, dysregula- tion of specific miRNAs has been implicated later in several pathologies including cancer. Owing to their presence and stability in body fluids, miRNAs have been investigated as novel circulating non-invasive biomarkers. Accordingly, their role as potential diagnostic, prognostic or predictive biomark- ers for many cancer types has recently emerged. This review tackles the use of circulating miRNAs in cancer detection, diagnosis and prognosis, giving examples using common solid tumors and discussing the advantages of their use, the challenges facing this novel circulating biomarker and recorn- mendatidns to overcome them.