Biological samples are three dimensional and, therefore, optical sectioning is mandatory for microscopic images to precisely show the localization or function of structures within biological samples. Today, researchers can choose from a variety of methods to obtain optical sections. This article focuses on structured illumination microscopy, which is a group of techniques utilizing a combination of optics and mathematics to obtain optical sections: A structure is imaged onto the sample by optical means and the additional information thereby encoded in the image is used to calculate an optical section from several acquired images. Different methods of structured illumination microscopy (mainly grid projection and aperture correlation) are discussed from a practical point of view, concentrating on advantages, limitations and future prospects of these techniques and their use in cell biology. Structured illumination can also be used to obtain super-resolution information if structures of higher frequency are projected onto the sample. This promising approach to super-resolution microscopy is also briefly discussed from a user's perspective.