This monograph contains Wenckebach's obserations on the physiological bases of arrhythmias, along with accounts of their clinical manifestations. Wenckbach was the first to describe extracystoles in man, demonstrating independently of Cushny (1906) that these ineffectual contractions were premature beats. His name is commemorated in the expression "Wenckebachs periods," denoting periodic dropped beats when a conduction defect exists.