MicroRNAs (miRNAs) are short non-coding RNAs involved in post-transcriptional regulation of gene expression. Since the discovery of the first miRNA gene lin-4 in C. elegans, over 500 miRNAs have been identified in humans and the list is growing. Their biological importance, initially demonstrated in cancer, was also more recently discovered in many other pathologies like Alzheimer's disease, Parkinson's disease, viral infections, diabetes, and myopathies. In the present review we will summarize miRNA profiling studies in human diseases and discuss the newly discovered link between microRNAs and drug response. An understanding of how microRNAs influence the body's response to certain drugs and how these affect the expression of microRNAs, will be of key importance in developing drugs with greater safety and efficacy.