This thesis is concerned with the use of zone plate interferometers for the precise location of position. The thesis begins by discussing the use of the zone plate as an optical component and examines the wavefront aberrations that can occur in higher orders of diffraction. Interferometers applicable to the measurement of position are then reviewed, including interferometers that incorporate optical zone plates as beam dividers and beam combiners. A zone plate interferometer for defining and locating multiple positions in three-dimensions is reported. Here, a three-dimensional array of positions is defined by a two-dimensional array of reflective zone plates. An optical probe containing two zone plates completes the interferometer, and is used to locate the positions. The displacement sensitivity of the probe can be controlled at the optical design stage. The variation of sensitivity with wavefront geometry is explored and expressions are derived for the transverse and axial sensitivity in terms of the conjugates of the illuminating wavefronts. Various wave front configurations were experimented with and the resulting position location patterns recorded. The measured values of sensitivity to displacement are compared to theory. A version of this interferometer was designed and developed by the author, for checking the performance of three-coordinate measuring machines used in engineering metrology. The results of trials with the interferometer, comparing the performance of measuring machines in several government laboratories, are reported. The averaged repeatability of setting, on one optically defined position, was found to be 1 μm in the transverse directions and 6 μm in the axial direction. Resulting from this work, two simple devices for pointing are described. Each uses a pair of zone plates to define an optical axis. In one case, the axis is precisely located by positioning a coherent light source and observing the interference bands generated by the device. In the other, moire bands are observed in incoherent illumination.