An ongoing challenge in cancer research is represented by the identification of new specific clinical molecular markers and pharmacological targets. During the last 10 years, microRNAs (miRNAs) have become one of the hottest subjects in the area of cancer genomics. MicroRNAs are single-stranded RNAs of 19 to 24 nucleotides in length generated through a complex maturation process. Recent studies have demonstrated that microRNAs can have an oncogene or tumor suppressor role by regulating the expression of target genes. Therefore, microRNAs are highly related to cancer processes, including initiation, growth, apoptosis, invasion, and metastasis. In this panorama, several high-through put technologies studies have revealed miRNA roles in classifying tumors and predicting patient outcome with high accuracy. We provide a review highlighting recent progress on the understanding of the cellular function of human microRNAs and their expression in solid tumors.