A general and simple method for 3-D image restoration in optical sectioning microscopy is presented. A set of images taken with an optical system with a known 3-D point spread function (psf) are considered. The restoration task is described, pointing out theoretical constraints on recovering the actual spatial distribution of the original object. A spatial-frequency analysis is performed, using no a priori information about the specimens udner investigation. The feasibility and limitations of 3-D reconstructions are analyzed. Moreover, a procedure is proposed that allows one to recover object projections within an angle range strictly dependent on the psf of the system used. 3-D objects have been simulated in order to produce a set of images similar to those provided by an optical system. Restoration has then been accomplished, and results demonstrate that side-views of an object can be obtained to a satisfactory degree of accuracy. Finally, a discussion concerning the presence of transparent and opaque objects in the field of view is also presented.