MicroRNAs are small non-coding RNAs involved in post-transcriptional regulation of gene expression. Thousands of miRNAs have been identified in different organisms including viruses, insects, plants and animals. MiRNAs has emerged as key regulators of important biological processes. The differential expression of miRNAs in various human diseases has made them potential candidates for developing novel therapies and personalized medicines. This book is focused on microRNA let-7, the second miRNA discovered in the year 2000 and one of the most studied miRNA. This book discusses various aspects of miRNA 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 signalling pathways in cancer, drug resistance and therapeutic potential in different human diseases.