This book sets-out to give the reader a non-mathematical understanding of the basic principles of migration and of building a velocity model of the earth's subsurface. The intended readership includes anyone who has to work with, or to understand, how contemporary seismic images are created: what are the underlying principles and pitfalls? How is a velocity model typically built and what are the consequences of not getting it right?Concepts such as uncertainty and non-uniqueness are discussed as are the ways in which these topics translate to risk-reduction and reliability in the final image. The different ways of representing a velocity model are reviewed as are the techniques used for picking velocity and anisotropy related information. A review of the principles of tomography is presented, to familiarize the reader with the techniques that underpin all contemporary velocity model update. Also, the physics behind anisotropy and its consequences for obtaining images in ?true? geological depth are discussed. An historical overview of velocity model building techniques over the past 30 years is presented to give the reader a feel for how the black art of model building has evolved in tandem with the increase in computer power and the emergence of powerful interactive graphics, covering the evolution from a purely linear compartmentalized industrial process towards a fully interactive multidisciplinary approach to iteratively building a reliable subsurface velocity model.The book concludes with a look at emerging and future trends: the promise of velocity-independent imaging and the potential of full waveform inversion.