Proper cardiac morphogenesis requires a series of specific cell and tissue interactions driven by several cardiac transcription factors and downstream cardiac genes. To date, a number of genetic aetiologies responsible for human congenital heart defects (CHDs) have been identified, although none has been found for CHDs in small animals. Most gene mutations responsible for human CHDs exist in genetic pathways associated with cardiomorphogenesis. Insights into cardiomorphogenesis from human and mouse genetic studies will help us to identify potential genetic aetiologies in CHDs in small animals. Therefore, in this first part of a two-part review, the major genetic pathways for cardiomorphogenesis and important candidate genes for CHDs, based on mouse knock-out and human genetic studies are discussed.