Genetics is traditionally defined as the science of biologic variation and has been a scientific discipline for over 100 years. Human genetics makes up a large part of the field of genetics, but the principal laws of genetics are universal, and apply equally to all species, including humans. Mendel’s studies in the nineteenth century were originally felt to have no relevance to humans, and it is only in retrospect that their importance can be seen. Many of the principles of genetics were discovered through the study of smaller organisms, such as bacteria, yeast, and fruit flies. The basic genetic mechanisms of cell division, development, and differentiation happen in the same way in widely divergent species. Therefore, it is impossible to look at human genetics in isolation, and there are large amounts of information from lower species that have bearing on human disorders. The study of the genetics of small organisms has had a profound impact on our understanding of human development and of how human diseases develop. It is likely that such basic science will continue to contribute significantly to the understanding of human genetic disease. This chapter will attempt to outline the basic elements of genetics, describe the types of genetic tests now available to help in neonatal diagnosis, and give an approach to the diagnosis of congenital abnormalities.