Individuals with higher-than-normal blood sugar levels used to be diagnosed as having either type 1 or type 2 diabetes. We now know that a wide range of different factors can cause diabetes, including single gene defects, which account for at least 1% of all diabetes cases and up to 4% of cases in the pediatric population. However, misdiagnosis remains common due to the considerable clinical overlap between the different diabetes forms. Monogenic diabetes onset can occur shortly after birth, as observed in neonatal diabetes mellitus, or any time later in life. The present chapter outlines the genes currently known to be involved in monogenic diabetes. Some of these genes are involved in β-cell development, with mutations often leading to a decreased β-cell number, while others play important roles in β-cell function and maintenance. Monogenic forms of autoimmune diabetes and epigenetic causes will also be discussed. A genetic diagnosis may influence treatment choice and prognosis determination and may also lead to family counseling. Genetic screening using next-generation sequencing is becoming more practical as it becomes increasingly accessible and less expensive.