Swedish biomedical researchers were important in the early development of the study of biological rhythms in the mid-20th century. This study looks at the foundation of biological-rhythm research (today called “chronobiology”). The first international society (Society for the Study of Biological Rhythm) was formed by a core group of Swedish scientists, who held their first meeting in Sweden, and who dominated the activities of this Society until the 1950s, when its membership became more international. Swedish researchers were therefore important for the emergence of this interdisciplinary field and for establishing its autonomy as a distinct field. The bulk of the book is a description of the early meetings of the International Society and the papers presented at those meetings, with biographical information on some of the key Swedish researchers.