The present invention generally relates to sub-diffraction limit image resolution. 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. 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. The emitted light may be used to determine the positions of the first and second entities, for example, using Gaussian fitting or other mathematical techniques, and in some cases, with sub-diffraction limit resolution. The methods may thus be used, for example, to determine the locations of two or more entities immobilized relative to a common entity, for example, a surface, or a biological entity such as DNA, a protein, a cell, a tissue, etc.