Seismology is the most powerful geophysical tool for understanding the structure of the Earth. It is concerned with how the Earth vibrates. Physically, seismic behaviour depends on the relationship between stress and strain in the Earth. ‘Seismology and the Earth’s internal structure’ explains compressional and shear elastic deformation and the four types of seismic waves caused by earthquakes: P-waves and S-waves that travel through the body of the Earth, and Rayleigh and Love waves that spread out at and near the Earth’s surface. It describes the reflection, refraction, and diffraction of body waves and how their observation and measurement by seismometers can be used to understand the internal structure of core, mantle, and crust.