Cardiac arrhythmias arise from disturbances in impulse formation or conduction, which can also occur in combination. Bradycardic arrhythmias result from impaired impulse formation in the sinus node or delays in conduction through the heart´s conduction system. The fundamental mechanisms for the development of tachycardic arrhythmias are considered to be increased automaticity, triggered activity, and reentry. This chapter is dedicated to the basics of cardiac impulse formation, impulse conduction, and the pathogenesis of arrhythmias. An understanding of these mechanisms allows for targeted diagnostics and therapy of cardiac arrhythmias.