The present invention generally relates to sub-diffraction limit image resolution and other imaging techniques, including imaging in three dimensions. In one aspect, the invention is directed to determining and/or imaging light from two or more entities separated by a distance less than the diffraction limit of the incident light. For example, the entities may be separated by a distance of less than about 1000 nm, or less than about 300 nm for visible light. In some cases, the position of the entities can be determined in all three spatial dimensions (i.e., in the x, y, and z directions), and in certain cases, the positions in all three dimensions can be determined to an accuracy of less than about 1000 nm. In one set of embodiments, the entities may be selectively activatable, i.e., one entity can be activated to produce light, without activating other entities. A first entity may be activated and determined (e.g., by determining light emitted by the entity), then a second entity may be activated and determined. The emitted light may be used to determine the x and y positions of the first and second entities, for example, by determining the positions of the images of these entities, and in some cases, with sub-diffraction limit resolution. In some cases, the z positions may be determined using one of a variety of techniques that uses intensity information or focal information (e.g., a lack of focus) to determine the z position. Non-limiting examples of such techniques include astigmatism imaging, off-focus imaging, or multi-focal-plane imaging. Other aspects of the invention relate to systems for sub-diffraction limit image resolution, computer programs and techniques for sub-diffraction limit image resolution, methods for promoting sub-diffraction limit image resolution, and the like.