Micro RNAs (miRNAs) represent a newly discovered class of regulatory molecules in the human body. miRNA is a short double stranded RNA sequence interfering with mRNA, causing in most cases, inhibition of translation. Synthesis of miRNAs shows an increasing developmental pattern and postnatally miRNAs are synthesized in all cells possessing transcriptional machinery. miRNAs usually target several mRNAs and therefore conclusive evidences proving their functions are not always ease to be acquired. In spite of this difficulty, functions of miRNAs were firmly established in the development, the cardiovascular and neural diseases, and cancer. Many miRNAs have been reported to be associated with physiological state of cells and/or tissues. This finding becomes fundamental, especially when consider that these miRNAs can be released from cell into intracellular space or circulation. Correlation between miRNA production in tissues and its contribution to multisource miRNA pool in the circulation is in a focus of biomarker-oriented researchers. Recently, circulating miRNAs have been suggested to be applicable as biomarkers in several types of cancer, cardiovascular injury, and diabetes. Role of miRNAs in the organism intercellular signaling is still under the broad investigation. Several miRNA mimics, intended for treatment of disease, are being currently tested in the clinical trials.