Current seismology depends on well-developed networks of instruments and a range of advances in both theoretical and computational developments. We provide a survey of the development of seismic recording, including the introduction of dense sets of portable instruments, so that major earthquakes are now captured by thousands of seismometers. We then discuss the way in which understanding of seismic waveforms has developed through the computation of synthetic seismograms and their exploitation in inversion. The last part of the chapter provides a description of the structure of the four Parts of the book