The inner core is a planet within a planet: a hot sphere with a mass of one hundred quintillion tons of iron and nickel that lies more than 5000 kilometres beneath our feet. It plays a crucial role in driving outer core fluid motion and the geodynamo, which generates the Earth's magnetic field. This book is the first to provide a comprehensive review of past and contemporary research on the Earth's inner core from a seismological perspective. Chapters cover the collection, processing and interpretation of seismological data, as well as our current knowledge of the structure, anisotropy, attenuation, rotational dynamics, and boundary of the inner core. Reviewing the latest research and suggesting new seismological techniques and future avenues, it is an essential resource for both seismologists and non-seismologists interested in this fascinating field of research. It will also form a useful resource for courses in seismology and deep Earth processes.