During the past decade, Nav1.5, the main voltage-gated Na+ channel in the heart, has been shown to be involved in many cardiac diseases. Genetic variants in the gene SCN5A, encoding Nav1.5, have been linked to various cardiac phenotypes, such as the congenital and acquired long QT syndromes, Brugada syndrome, conduction slowing, sick sinus syndrome, atrial fibrillation, and even cases of dilated cardiomyopathy. This unexpected phenotypic diversity may reflect that Nav1.5 is not only restricted to the initiation of the action potential and rapid cardiac conduction, but may also be involved in other, not-yet elucidated, functions. Despite the fact that our understanding of the regulation of expression, localization, and function of Nav1.5 is deepening, we are still far from a comprehensive view. Much of our current knowledge has been obtained by carrying out experiments using “cellular expression systems”, e.g. host cells expressing exogenous Nav1.5. Although very informative, these techniques are limited, in that Nav1.5 is not expressed in the physiological cellular environment of a cardiac cell. Recently, however, there have been several studies published which used approaches closer to “normal” or pathological physiology. In an attempt to summarize recently published data, this article will review the phenotypes of genetically-modified mouse strains where Nav1.5 expression and activity are directly or indirectly modified, as well as the regulation of Nav1.5 function using native cardiac myocytes. Despite obvious limitations, the reviewed studies provide an overview of the complex multi-factorial and multi-protein regulation of Nav1.5.