Over the past 2 decades, investigators in the field of cardiac genetics have evolved a complex understanding of the pathophysiological basis of inherited cardiac diseases, which predispose individuals to sudden cardiac death. In this Review, we describe the current status of gene discovery and the associations between phenotype and genotype in the cardiac channelopathies and cardiomyopathies. The various indications for genetic testing and its utility in the clinic are assessed in relation to diagnosis, cascade testing, guiding management, and prognosis. Some common problems exist across all phenotypes: the variable penetrance and expressivity of genetic disease, and the difficulty of assessing the functional and clinical effects of novel mutations. These issues will be of particular importance as the next-generation sequencing technologies are used by genetics laboratories to provide results from large panels of genes. The accurate interpretation of these results will be the main challenge for the future.