Use of conventional microelectrodes allows recording of the diverse shapes of human cardiac action potentials, each of which is characteristic for the various locations within the heart. The single cell voltage clamp technique enables analysis of the underlying ionic current and molecular biology and genetics provide tools to study expression and modification of ion conducting channels including their regulatory subunits. Atrial and ventricular cardiac tissue change their structure and function in response to disease (remodeling) Pathological expression and function of ion channels is relevant in clinical medicine since they can lead to life-threatening arrhythmias in particular when the ventricles are involved. Concentrating on repolarising ionic currents I will give a brief overview of the diversity of changes encountered in the diseased heart with particular emphasis on atrial fibrillation and heart failure. During remodeling, not only the expression level of ion channels but also their electrophysiological properties can significantly change. In addition, their responsiveness to drugs can become modified, where gain or loss of function may be similarly deleterious.