This book is primarily intended for seismologists and technicians working with seismological instruments. The aim is that it should be used as a practical tool with only the amount of theory needed to understand the basic principles, and the answers to most practical problems should be found here. This book is written in such a way that it is understandable for a broad range of professionals working with seismological instruments and seismic data, whether students, engineers, or seismologists. Both practical and theoretical topics are covered, with the main emphasis on practical aspects. The book is based on the authors' (seismologist and physicist) experience working in this field for more than 25 years.