A review is given of the fundamental aspects of the phase retrieval problem in optical imaging for one dimension. The phase problem is treated using the fact that the wavefunction in the image‐plane is a band‐limited entire function of order 1. The ambiguity of the phase reconstruction is formulated in terms of the complex zeros of entire functions. Procedures are given how the relevant zeros might be determined. When the zeros are known one can derive dispersion relations which relate the phase of the wavefunction to the intensity distribution. The phase problem of coherence theory is similar to the previously discussed problem and is briefly touched upon. The extension of the phase problem to two dimensions is n o t straight‐forward and still remains to be solved.