This book constitutes one of the most challenging problems in Mechanical and Civil Engineering, crossing very different applications and scales, from biomechanical and mechanical components to long span bridges and aerospace structures. It is the 2nd contribution from the same authors for the book series Measurement of Physical and Chemical Quantities, focusing on techniques and sensors to measure physical and chemical quantities.Addressed to researchers with different backgrounds, the book presents in an accessible and concise form the various physical principles and associated techniques for displacement measurement. These comprehend the most classical techniques, based on the measurement of variations with displacement of electrical resistance, capacitance and inductance, or else optical techniques using electromagnetic, acoustic or interference wave measurements, still with open fields of research and offering a wide range of applications.