The goal of this paper is to provide an overview of the current state of the art in seismic tomography, and trace its origins from pioneering work in the early 1970s to its present status as the pre-eminent tool for imaging the Earth's interior at a variety of scales. Due to length limitations, we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles; rather, we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic. The relative merits of local versus global parameterization, ray tracing versus wavefront tracking, backprojection versus gradient based inversion and synthetic testing versus model covariance are explored. A variety of key application areas are also discussed, including body wave traveltime tomography, surface wave tomography, attenuation tomography and ambient noise tomography. Established and emerging trends, many of which are driven by the ongoing rapid increases in available computing power, will also be examined, including finite frequency tomography, full waveform tomography and joint tomography using multiple datasets. Several practical applications of seismic tomography, including body wave traveltime, attenuation and surface waveform, are presented in order to reinforce prior discussion of theory.