A method for providing an image of a sample, the method comprising: providing first activation radiation to a sample that includes phototransformable optical labels PTOLs, the PTOLs being distributed in at least a portion of the sample with a density greater than an inverse of a diffraction limited resolution volume of imaging optics, and the PTOLs being transformable from an inactivated state wherein the labels radiate little, if any, radiation at a wavelength characteristic of an activated and excited PTOL when exposed to excitation radiation, to an activated state wherein illumination by excitation radiation results in detectable emission of radiation to activate a first sparse subset of the PTOLs in the sample; controlling the first activation radiation such that the density of the activated PTOLs in the first sparse subset is less than the inverse of the diffraction-limited resolution volume of the imaging optics; providing first excitation radiation to the first sparse subset of PTOLs in the sample to excite at least some of the activated PTOLs; detecting with the imaging optics fluorescence radiation (305, 307) emitted from activated and excited PTOLs within the first sparse subset of PTOLs and determining the locations of the activated and excited PTOLs with superresolution accuracy; deactivating the activated PTOLs and repeating the above steps to determine the locations of more PTOLs within the sample; and combining the determined locations of all the PTOLs from the different images to build up a superresolution image of the sample, wherein providing the first activation radiation to the sample activates PTOLs in the first sparse subset of PTOLs in the sample through a multi-photon absorption process and/or wherein providing the first excitation radiation to the sample excites PTOLs in the first sparse subset of PTOLs in the sample through a multi-photon absorption process. providing first activation radiation to a sample that includes phototransformable optical labels PTOLs, the PTOLs being distributed in at least a portion of the sample with a density greater than an inverse of a diffraction limited resolution volume of imaging optics, and the PTOLs being transformable from an inactivated state wherein the labels radiate little, if any, radiation at a wavelength characteristic of an activated and excited PTOL when exposed to excitation radiation, to an activated state wherein illumination by excitation radiation results in detectable emission of radiation to activate a first sparse subset of the PTOLs in the sample; controlling the first activation radiation such that the density of the activated PTOLs in the first sparse subset is less than the inverse of the diffraction-limited resolution volume of the imaging optics; providing first excitation radiation to the first sparse subset of PTOLs in the sample to excite at least some of the activated PTOLs; detecting with the imaging optics fluorescence radiation (305, 307) emitted from activated and excited PTOLs within the first sparse subset of PTOLs and determining the locations of the activated and excited PTOLs with superresolution accuracy; deactivating the activated PTOLs and repeating the above steps to determine the locations of more PTOLs within the sample; and combining the determined locations of all the PTOLs from the different images to build up a superresolution image of the sample, wherein providing the first activation radiation to the sample activates PTOLs in the first sparse subset of PTOLs in the sample through a multi-photon absorption process and/or wherein providing the first excitation radiation to the sample excites PTOLs in the first sparse subset of PTOLs in the sample through a multi-photon absorption process.