Micro RNAs are small non-coding RNAs involved in posttranscriptional regulation of gene expression. Thousands of mi RNAs have been identified in different organisms including viruses, insects, plants and animals. Mi RNAs has emerged as key regulators of important biological processes. The differential expression of mi RNAs in various human diseases has made them potential candidates for developing novel therapies and personalized medicines. This book is focused on micro RNA let-7, the second mi RNA discovered in the year 2000 and one of the most studied mi RNA. This book discusses various aspects of mi RNA let-7 starting from its discovery, biogenesis, transcriptional and posttranscriptional regulation to its crucial role in various fundamental cellular processes such as development, stem cell maintenance and differentiation, regulation of signaling pathways in cancer, drug resistance and therapeutic potential in different human diseases.