A method for determining construction parameters of magnet structures designed to generate magnetic fields in predetermined volumes of space, comprising the steps of: a) defining a predetermined volume of space in which a static magnetic field is to be generated; b) defining the nominal strength and profile of the magnetic field in said predetermined volume of space; c) mathematically describing the nominal magnetic field on the surface of and/or in said predetermined volume of space by using a polynomial expansion of the magnetic field-describing function, which provides a series of coefficients; d) determining the size, geometry and relative position in space of the means for generating said nominal magnetic field, with respect to the volume of space in and on which the desired, i.e. nominal magnetic field, is defined. According to the invention, the predetermined volume of space is delimited by an ideal ellipsoidal closed surface and the magnetic field is expressed by coefficients of a polynomial ellipsoidal harmonic expansion. The method may be also used as a shimming method in volumes of space ideally delimited by ellipsoidal closed surfaces, to be implemented whether or not the predetermined volume of space defined by the magnet structure is also delimited by an ideal closed surface, with an ellipsoidal or other shape.