Abstract: The present invention generally relates to sub-diffraction image resolution and other imaging techniques. 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 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 positions of the first and second entities, for example, using Gaussian fitting or other mathematical techniques, and in some cases, with sub-diffraction resolution.