The concept of monogenic diabetes emerged 25 years ago with a paper reporting the glucokinase locus linkage to maturity-onset diabetes of the young, an autosomal dominant disorder of glucose metabolism. Since then a huge leap forward has been made with the discovery of other clinical forms of monogenic diabetes, such as neonatal diabetes mellitus (NDM), and the identification of literally tens of genes that cause diabetes, either in isolation or syndromic. Of note, NDM genetics not only shed light on several aspects of pancreatic β-cell biology, but revealed new and unexpected therapeutic options for patients carrying mutations in specific genes, such as oral hypoglycemic agents (mutations of KATP genes) or stem cell transplantation (IPEX). In this chapter, we describe the genetic defects leading to neonatal diabetes that recognize dominant, recessive, and X-linked modes of inheritance or the association with incorrect parental origins of chromosomes and disturbances of imprinting.