Due to the nature of interference phenomenon, optical interferometry allows for very precise measurements to be achieved. Recent discoveries extended the purpose and precision of interferometry and made it more practical and applicable. It was also revolutionized by intensive usage of photodetectors and digital signal processing electronics. Both, specular and speckle interference were used to investigate displacements, vibrations and wavefronts by modern interferometric techniques. The book provides qualitative and quantitative results on vibration measurement of specular and rough objects with non-contact methods, the principles and development of a quadrature interferometer for displacement measurement with improved resolution, and a self-referenced method based on lateral shearing interferometry for wavefront measurement. This work is addressed to researchers and engineers with an interest in measurement and characterization science by high resolution and non-invasive methods. It may also be useful to professors and students as a notebook on optical interference phenomenon and its nowadays applications.