The objective of this review is to evidence the potential of micro-RNAs (mi-RNAs) as possible diagnostic biomarkers in different diseases. Micro-RNAs are nucleic acids of 22 nucleotides that regulate the translation of coding messenger RNAs (mRNAs), and produce a post-transcriptional regulation of genetic expression. Most mi-RNAs are highly conserved and show a tissue-specific distribution; therefore, they play an important role in the regulation of cell function and the physiopathology of different organs. Micro-RNAs emerge as potential candidates for biomarkers due to the changes in their levels of expression in different situations (cancer, hepatic and cardiovascular diseases) and in fluids such as plasma, serum, urine and saliva. However, although some mi-RNAs have a consistent expression profile, others have been reported as possible biomarkers for more than one disease, thus limiting their specificity and usefulness as diagnostic tools. Further studies are important to define the significance of mi-RNAs in pathologic processes and their role as possible biomarkers.