The genome is the totality of information that directs the making and the maintenance of you and every other living organism. Scattered among the familiar genes that code for the proteins of life are other genes. This is a book about the genes we call microRNA. It is 30 years since their discovery. They are gene regulators, every bit as vital as their more famous gene cousins. MicroRNAs fine-tune how much protein is made in our cells, each one coordinating the activity of hundreds of genes and bringing precision to the ‘noise’ of gene expression. Without them, life is virtually impossible. This introduction provides a personal account of what fascinated the author about these genes enough to make him redirect his research to microRNAs. The journey from studying pharmacology in the UK, to the USA where his interest in the brain disease epilepsy began, and later to Dublin, to work at the Royal College of Surgeons in Ireland. It lays out the contents and style of the book, which is part history of science, describing what we know and the experiments that underpin our understanding, and part memoir of the author’s own research, and the applications of microRNAs in medicine.