The purpose of this book is to give a popular review of modern seismology, its research methods, problems of current interest and results and also to some extent to elucidate the historical background. Especially in recent years, seismology has attracted much interest from the general public as well as from news agencies. The reasons for this are partly con- nected with recordings of large explosions (nuclear tests), partly related to earthquake catastrophes. This interest and the questions which people have asked us for the past years have to a certain extent served as a sti- mulus in the preparation of this book.