The contraction pattern of the cardiac chambers is controlled by myocytes of the pacemaker and the conduction system. In the developing heart all cardiac myocytes initially possess pacemaker properties. However, the majority of the embryonic myocytes differentiates into working myocardium, and only a small population of these cells will form the sinus node and the atrioventricular (AV) node and bundle. The development of these nodal regions is achieved by highly localized suppression of working muscle differentiation and stimulation of the pacemaker gene programs. Key transcription factors that control this process have been identified, including T-box transcription factors Tbx3 and Tbx5. During development, the AV canal myocardium is responsible for the AV conduction delay. This type of myocardium is highly evolutionary conserved in vertebrates and will form the AV node in higher vertebrates like birds and mammals. Recently, we identified a distinctive sinus node cell population in the fish heart, revealing deep conservation of these cells as well. Despite the electrophysiological similarities between myocytes of the sinus node and the AV node and conduction system, their molecular programs show striking differences. For example, Tbx18, Shox2 and Isl1 uniquely control sinus node development, whereas Tbx2 and Nkx2-5 control AV canal formation. In this presentation, I will show experimental findings on the molecular mechanisms of differentiation and morphogenesis of the pacemaker tissues of the heart, and discuss how misregulation of AV canal development can lead to formation of accessory pathways and ventricular preexitation.